The ABC's of Running High Boost for the
1991 200TQ 20V Turbo and the 1992-96 Audi S4/S6

It should be your responsibility (or your mechanics) to check out and verify all the ECU and Fuel Injection system components mounted on your engine are working correctly BEFORE you do any type of modification that will raise the boost on your vehicle.

Raising the boost on a high mileage car isn't a great idea UNTIL you get the stock system working correctly as it was originally designed when it left the factory. If you install a stiffer Waste Gate spring or use a valve connected to boost pressure to add pressure on top of the Waste Gate Diaphragm, you may be unaware if the engine ECU/Fuel system components are not working correctly, because the boost may be raised regardless of the condition of the stock system components.

Whenever I purchase a used Turbocharged Audi, I check the compression or do a leak down pressure test to verify the engine valves, rings, and head gasket are in good shape. This leak down test is better than a basic compression test and involves pressurizing the cylinder with air and measuring the amount of leakage out of the cylinder.

This can pin point cylinder leakage out of the intake or exhaust valves, past the rings, or through the head gasket. I replace the plugs, air filter, and fuel filter and change the engine oil. I check/replace the O2 sensor. I replace the valve cover gasket and any suspect breather or vacuum hoses.

I make sure the timing belt has been replaced at the suggested 60-90K interval. I remove the lower Intercooler/turbo hose(s) and inspect them, I check/replace the accordion style hose at the Intercooler connection. It is also very important to check/replace the turbo bypass valve.

On the 1991 200TQ, it is important to Check/Replace the small Bypass valve hose (vacuum/boost hose) that connects from the bypass valve to the small fitting at the back of the intake manifold. On the 1992-95 S4/S6, you may want to remove the black plastic firewall cover, and replace the crimped on hose clamps at the ECU vacuum/pressure hose where it connects to the clear moisture trap (Looks like a empty fuel filter) as these hose can blow off as the boost goes up.
You can also pressure test the intake/boost hoses, by making an adapter that is inserted into the hose between the MAF and the turbo, and then apply regulated compressed air (~15psi) and pressure test the hoses from the turbo inlet, out the turbo exit hose, through the Intercooler, and into the throttle. If you are running the 1+ type ECU mods, which produce over 20psi of boost, using the Samco boost hoses may save your screamin K24 turbo some day.

I check the operation of the idle/potentiometer throttle switch, and I remove and clean the idle stabilizer valve. I check the ECU measurement block values (10 values) to ensure the ECU is operating the engine as expected. If a problem with the mixture adjustment shows up, the intake vacuum hoses, or the fuel pressure may need to be checked.

The cooling system integrity should be accessed. Replacing the thermostat, and all the coolant hoses, the heater control valve, the heater core to engine hoses, the radiator fan temperature sensor, the after-run temp sensor, and the Multi-Function temperature sensor is also a good idea on vehicles with over 100k miles.

You don't want a $10 hose ruining your expensive 20V engine when the hose blows and causes overheating and a possible blown headgasket! Once I complete the replacement of these many high mileage components, and ensure the engine/ECU is operating correctly, I can rest assured, and I don't need to "guess" if they are ok if the car starts running poorly.

Don't get lazy and "assume" these components are ok, just bite the bullet and inspect or replace them as necessary. NOTE: Increasing the boost on your vehicle may change the emissions output which may not be legal in your area.
Stock Boost Levels-How does the ECU control Boost?

The 1991 200TQ with 20V Turbo Engine was designed to run a maximum of 1.825 bar boost (absolute) or ~12 psi gauge pressure, as set by the Motronic Engine Control Unit (ECU).

The 1992-96 Audi S4 in overboost mode will have boost set to about 2.15 bar absolute. The actual boost produced will vary depending on engine RPM, air temperature, fuel quality, and altitude. The 20V boost control system is different than what was used on the earlier 10V Audi Turbo engines.

The ECU uses the Waste Gate solenoid to send boost to the lower chamber in the waste gate to open up the waste gate at the appropriate boost level. The Waste Gate solenoid valve is designed to be open with no electrical signal applied, which will send turbo boost pressure directly to the lower chamber in the waste gate and open the waste gate at a pressure only determined by the stiffness of the waste gate spring.

When the waste gate solenoid is energized with a high duty cycle, (more on time) the waste gate solenoid will prevent boost pressure from reaching the waste gate lower chamber, and allow the boost pressure to climb higher. The boost will climb, until the required boost level set in the ECU is reached, at which point, the duty cycle percentage (%) to the waste gate solenoid will be reduced, so the boost pressure will once again be sent to the lower chamber in the waste gate diaphragm and the boost pressure will be controlled. In the stock 20V engines, this corresponds to about 1.4-1.5 bar of boost as controlled by the waste gate spring. If you completely remove the lower waste gate hose, or block it off, the waste gate will only open when "exhaust" pressure acting on the waste gate valve face, exceeds the waste gate spring pressure.

This is why a modified 20V ECU can make 2.25 bar absolute boost pressure, even with the stock waste gate spring installed. The ECU uses a barometric sensor to monitor the altitude the vehicle is at, and will trim back the boost slightly as the altitude increases above 3900 feet. For example, in the 1991 200TQ 20V, the boost will be reduced to about 1.715 bar absolute, when the altitude is above 8500 feet [1].

Stock Digital Boost Gauge, Aftermarket Boost Gauge, Pressure Conversion Info.

The stock digital boost gauge located in the instrument cluster displays the intake manifold "absolute" boost pressure, from ~0.0 bar to 2.0 bar. The S4 boost gauge will read up to 2.5 bar on the 1992 models.

Some of the later Canadian S4 models also have the boost gauge.

For more details, go to the Stock Boost Gauge page

Self Diagnostic System-Fault Codes

The self-diagnostic system test should be run before you have your stock ECU modified, to make sure that there are no faults in any of the components (knock sensors, coolant/air temp sensors, throttle switch etc.) that the ECU relies on for correct engine operation. You need to do a 3rd gear full throttle run to exercise this diagnostic system correctly.

The 1991 200TQ's with the Bosch Motronic ECU, have a non volatile memory for the fault codes that can be read later even if the engine has been turned off after the test drive. The ECU system OUTPUT tests should also be done on the stock ECU before doing any modifications to verify that the Wastegate Solenoid, the Carbon Canister valve, the idle stabilizer valve and the fuel injectors are working correctly.

In some cases the ECU internal drive transistors for these solenoids can be defective and will prevent correct operation of the engines fuel and boost control systems. If there is a problem with any of the solenoids or the internal operation of the ECU, this needs to be known BEFORE you send in a ECU for modification.

Go to the 20V ECU Fault Code page at this web site to run these Output tests.


Boost Problem Checklist

Here is a checklist you can go through when you have low or high boost problems.
LOW BOOST or Running poor issues
  • Check for any stored ECU fault codes, are the knock sensors working ok?
  • Read the Measurement block values using the VAG-COM, look for anything out of the range
  • Check basic engine timing, flywheel, camshaft and distributor rotor position
  • Check Throttle Cable Adjustment
  • Check Air Filter, is it really clogged up?
  • Check/Replace the Turbo Bypass Valve, check small vacuum/boost hose to this valve
  • Check Idle Stabilizer Hoses, and other small vacuum/boost hoses
  • Check basic fuel pressure and correct operation of the rising rate fuel pressure action by applying vacuum or boost to the vacuum/boost hose connection
  • Check fuel pressure regulator for leaking into the vacuum/boost hose fitting (rich running)
  • Check ECU moisture trap hoses (fuel filter lookin thang) behind black firewall cover (AAN S4/S6)
  • Check ECU Connector Terminals for corrosion, remove connector and use contact cleaner
  • Replace small vacuum hose from Bypass valve to Intake Manifold (3B engine)
  • Check the Operation of the Multi-Function Temp Sensor (1991 200Tq 20V Only)
  • Temporarily disconnect Multi-Function Temp Sensor 4 pin connector and recheck boost output (1991 200Tq 20V Only)
  • Remove and inspect the Intercooler exit hose (Ribbed Hose) and the lower turbo exit pressure hose for leaks.
  • Check the rubber hose between the intake pipe and the throttle body.
  • Check Vacuum hoses and their connections at the back of the intake manifold
  • Check/Replace the coolant thermostat, is the engine running too cold? or too hot?
  • Check Engine Coolant Temperature Sensor Resistance and connector wiring
  • Check the operation of the Waste Gate Solenoid using the ECU Output Test
  • Check Waste Gate Solenoid hoses
  • Check Air Temp sensor resistance at the ECU, and Repair the Air Temp Sensor wiring connections if needed.
  • Check Throttle Position Switch/Potentiometer for intermittent operation
  • Check Altitude Sensor Voltage Output and wiring between sensor and ECU
  • Plug or pinch off hose to lower waste gate and check for maximum boost and overboost cutout
  • Check Waste Gate for sticking open, defective spring?
  • Check the Exhaust System Back pressure, possible exhaust restriction
  • Inspect the Turbo inlet vanes and end play, has the turbo compressor nut left the building? or the turbo seized up, broken in half, or otherwise gone south?
  • Check Lower Waste Gate Hose from Intake Manifold for leaks/restrictions
  • Check the operation of the Waste Gate Solenoid using the ECU Output Test
  • Check Vacuum/Boost Hose from Intake Manifold to Engine Control Unit (ECU) for leaks
  • Pressure test the intake/boost hoses using hose adapter and compressed air regulator adjusted to 15psi.
  • Check and clean the electrical connections at the MAF sensor
  • Check Waste Gate Diaphragm for Leaks
  • Check Waste Gate Valve for sticking or restricted movement

Overboost Cutout?

ECU fault code 2224? If you are encountering an overboost condition which is causing the ECU to cut the fuel pump, and you have stored the 2224 ECU fault code, here are some items to check out.

You may have a leaking wastegate Diaphragm, test the wastegate diaphragm for leaks below.

You could also have a leak in the boost hoses between the turbo exit connection and the throttle body.

If you have a hand pump or an air compressor, you can make up an adapter to fit into the hose that connects to the turbo inlet, and this will allow you to pressurize the air from the turbo inlet, through the turbo exit hose, through the intercooler, from the intercooler exit hose, and from the metal intake pipe to the throttle body and check for any leaks. Use a pressure regulator set to about 15psi if you are using an air compressor.

You can also do a visual check of the turbo exit hose underneath the car and look for any signs of oil leaking out of the turbo exit hose, indicating a leak. Normally a small amount of oil vapors and mist are sucked in by the turbo from the crankcase breather system, and this oil mist will typically show up outside the hoses, or intercooler connections if a pressure leak is occurring.

Check and or replace the ribbed intercooler exit hose as well, these often split open on the bottom side, in-between the wire re-enforced rib areas, due to the crankcase breather oil mist in the system. Removing the hoses and inspecting them is also a good idea on high mileage engines.

If you don't have any leaks in these hoses, check the vacuum/boost hose that connects from the back of the intake manifold to the ECU inside the car.

See below for details.

I have run across a couple of 20V Turbo cars which had a 2224 code stored inside the ECU and were getting the overboost fuel pump cutout, even though the boost was only going up to 1.0-1.2 bar.

In the first case this would occur only when the throttle was quickly floored, and would not happen if the throttle was gradually opened all the way. In the first case, it ended up being a defective MAF sensor. In the other case it was only a poor connection at the MAF sensor.

Try using some electronic contact cleaner on the MAF connector and terminals, and see if the overboost problem is eliminated.

Waste Gate Controls

The Waste Gate solenoid connecting hoses should be checked for leaks and the ECU Waste Gate Solenoid "Output Test" should be run on the Waste Gate Solenoid, as these solenoids are often found to be defective on the 20V engine.

Go to 20V wastegate controls section for more details.

NOTE: On the 20V turbo 5 cylinder engines, there is NOT supposed to be any hose connected to the top of the waste gate cap fitting. It should be left open to the atmosphere.


You can use a hand vacuum pump to test the waste gate diaphragm for leaks. Connect the hand pump to the top waste gate chamber port and suck it down to about 15-20in vacuum.

The waste gate upper chamber should hold vacuum. If it is holding vacuum, you can quickly remove the hose and you "may" hear the waste gate valve "CLUNK" back down to verify the waste gate is not stuck.

The waste gate valve normally moves about 15mm when it is fully open.

Go to the Waste Gate Spring Installation page for details on inspecting the WG diaphragm if you suspect you have a leak.

In some cases if a stiffer waste gate spring has been installed incorrectly with no upper perch support, the waste gate valve shaft will operate with too much force to the side and can wear out the wastegate valve guide. This worn out guide can allow boost leaks through the waste gate.

Engine Control Unit (ECU) Vacuum/Boost line

In order for the ECU to work correctly, it must receive an accurate pressure signal (vacuum or boost) from the intake manifold. There is a plastic line with rubber hoses on each end connected from the ECU to the intake manifold.

The vacuum/boost line (Green plastic line and rubber hose) from the intake manifold to the ECU should be checked for leaks if you are getting too much boost or a fault code 2221, 2222 or 2224. You can connect a hand vacuum pump to the line after removing it from the back of the intake manifold and see if it holds vacuum. If you have access to compressed air and have an accurate pressure regulator, you can apply 10-15psi to the vacuum/boost line and listen for any leaks.

If you turn on the ignition, (Don't start the engine) you can read the boost pressure off the digital instrument panel boost pressure display, and see if it corresponds to the actual pressure applied to the ECU line from the air pressure regulator. Be careful when applying pressure to this line, and avoid exceeding 20 psi or you could damage the internal ECU pressure sensor.

The 1991 200TQ 20V Motronic ECU has the moisture trap located next to the ECU underneath the kick panel cover that can leak. The S4/S6 has the moisture trap located underneath the plastic cover on the firewall at the back of the engine compartment.

It is very common for the hose to get blown off the moisture trap on the S4/S6 vehicles and cause the vehicle to have low boost and run poorly in the "Limp Home" mode. The S6 may have the moisture trap near the ECU. I recommend replacing the crimped on clamps with conventional screw type hose clamps at this moisture trap location.

Turbo Bypass Valve and Intercooler leaks

This valve should be removed and checked as this can cause a loss in boost pressure if the internal diaphragm is leaking. Use a handheld vacuum pump connected to the small hose fitting to check this valve operation. It should hold vacuum and open and close freely.

This bypass valve relies on boost pressure acting on this internal diaphragm to hold the valve closed when the engine is making positive manifold pressure (above 1.0 bar absolute boost pressure) so you should check the small hose connected to this valve.

The 1991 Audi 200TQ 20V often had problems with the vacuum/boost supply hose for this bypass valve getting burned/toasted as the hose was run along side the exhaust manifold area. Remove the heat shield and inspect/replace the hose as needed.

Rerouting this hose away from the exhaust area is a good idea. This bypass valve opens during manifold vacuum conditions (below 1.0 bar manifold pressure) to allow intake air to circulate around the turbo cold side impeller.

This bypass valve helps to reduce turbo lag and keep the turbo spinning during transmission shifting when the engine is producing boost. It also helps to prevent boost pressure spikes from causing leaks or damage to the intercooler or connecting hoses.

Audi Bypass Valve Info:

The 1991 200TQ 20V with 3B engine originally used a bypass valve with the Audi part number 034 145 710 which had the metal reinforcement plate on the diaphragm, this bypass valve may have had the Bosch 0 280 142 106 part number on it.

An Audi Service bulletin: Group 21 Number 00-01 Dec. 8, 2000 was issued that described a problem with the Audi TT's that were equipped with the ATC, AMU and AWP code 1.8T engines. A groaning, howling or rattling type noise may be heard to come from the bypass valve.

A new Bosch bypass valve was introduced (Audi # 06A 145 710N) to address these complaints. There is a Bosch equivalent part number for this new valve, and it is reported to be a good replacement valve for other Audi turbocharged engines utilizing a bypass valve.

Note: The normal failure with these plastic Bosch bypass valves, is the rubber diaphragm develops a tear, either near the outside edge of the diaphragm, or at the sealing surface.

A few aftermarket performance companies have a new all metal, version of the bypass valves, one is called the "Bailey" valve, and one I believe is called the Forge valve. I have not tested these valves, so I can't comment on their effectiveness. The intercooler end cap seals can leak under high boost and you should install some large straps or one or two large hose clamps around the intercooler to hold it together under high boost.

Many hydraulic supply houses carry the screw type hose clamps that are 4 ft long which fit around the intercooler nicely. Because there normally is an oil mist flowing through the intercooler, any leaks around the intercooler seals will show up as oil seepage out of the intercooler end caps.

In some cases you can carefully re-crimp the aluminum tabs that hold the plastic end tanks on to tighten up the end cap seal. The internal intercooler rubber seal on the inlet side that seals the internal upper and lower sections of the intercooler should be checked when you remove the hoses from the intercooler.

Here is a picture of the larger single pass intercooler from the 1991 200TQ 20V.
There is an O-Ring on the inside of the intercooler inlet on these intercoolers that seals this pipe to the intercooler. This O-ring should be checked for leakage and replaced if necessary.

The Audi Part number is N 904 324 01, designated as a round seal 70X4N

Boost Hose Leaks, Wastegate valve boost leaks, Turbo intake hose

Often times on high mileage (100k>) cars, the turbo hoses begin to leak boost pressure from loose hose clamps and in other cases when running higher boost above the stock 12psi (1.8 bar absolute) levels, the original boost hoses can tear and split open.

You can make a pressure test cap to pressurize the intake/boost hose system to 15-20psi and look for any leaks from the turbo intake hose, to the turbo exit hose, from the intercooler to intake ribbed hose, into the throttle valve and finally to the intake manifold. You should remove the oil cap and pinch off the breather hose at the intake manifold connection to avoid pressurizing the crankcase.

This test cap can be made using a 3 inch black plastic sewer pipe cap, and a bolt in tire valve stem. You insert this test plug into the turbo intake hose after removing the hose from the MAF sensor. You may need to use a pressure regulator with your air compressor to limit the pressure of the system to 15-20psi.

Common boost leaks: 1991 200TQ: The accordion-style hose that connects the intercooler exit to the intake manifold normally rots (from oil) over time and can split at the bottom between the ribs while under high boost. On the later 1992-95 S4, the ribbed hose from the boost pipe into the intercooler underneath the engine will often split in the ribbed area for the same reason.

Remove and check them. If your car has over 100,000 miles on it and this hose has never been replaced, save yourself some grief and buy a new hose. Samco has some heavy duty silicone replacement boost hoses for the Audi 20V's.

The turbo exit hose can also split on the 1991 200TQ 20V and on the S4's where the bypass valve hose is melded into this turbo exit hose. Many folks are running over 20psi with the stock K24 Turbo, and if you get a severe boost hose leak, you can over-spin the K24 into destruction.

In some rare cases the Wastegate valve guide will be worn excessively and will leak boost into the exhaust system. This can be caused by using stiff wastegate springs with a loose or poorly mounted upper spring perch in the wastegate cap.

Turbo Intake Hoses:
The large turbo intake hose with connections to the bypass valve, breather pipe and Mass Air Flow sensor should be removed and checked for any cracks as this will allow un-metered air inside the engine.

While you have the intake hose off the turbo, you may also want to check the cold side turbo shaft for excessive play. The K24 turbo Audis spec the max end play to be 0.00354 inches, and the cold side compressor side play to be no more than 0.0259 inches. Normally you need a dial indicator to accurately measure this.

You should also look for any metal shrapnel caused by worn turbo bearings which may have allowed the turbo compressor blade to rub against the turbo housing. The turbo exit hose (turbo pressure) going to the intercooler should be removed and checked as well. The 1991 200TQ 20V has a metal boost pipe that is inserted into the intercooler inlet uses an O-Ring for sealing this connection, see section below for details if you see evidence of a leak at this point.

Vacuum Leaks-Breather Hoses!

You need to make sure your engine’s various breather/vacuum hoses and turbo intake and pressure hoses are in good shape. Leaks anywhere in these hoses can cause lean or rich running and poor starting. Vacuum leaks would typically cause lean running, pressure leaks after the turbo could cause slightly rich running. In some cases it is not sufficient to just visually look at these hoses without first removing them!

On the 1991 200TQ 20V with the 3B designated engine, the large breather hose coming from the side of the engine that is routed around the back of the engine typically gets rotten and mushy on the inside part of the hose facing the engine block.

Often times the hose can collapse on itself or will split open on the back side. This hose does not have hard vacuum applied inside like the 10V Audi Turbo MC engine but leaks could allow un-metered air into the engine.

Here is a photo of the 1991 200TQ 20V breather hose, Audi part # 034 103 221AF, Pressure or flow regulating valve (034 129101A), check valve or what Audi calls the bleeder valve (035 103 245A) , and the small special molded hose (034 133 367C) , that connects to the intake manifold and the carbon canister solenoid valve.

The spiral wire brush flame deflector inside the hose has Audi part number 035-103-477A Always check with your Audi dealer parts person to verify these numbers.

This 1991 200TQ 20V breather hose is routed around the back of the engine and one end connects to the round black crankcase pressure regulator valve near the back of the valve cover.

This black regulator valve, controls the flow of crankcase fumes to the turbo intake boot [1]. The valve has two ports inside, one is approximately 0.400 diameter and a smaller one that is approximately 0.180 inches. The larger port and the smaller port are open during low boost operation but when the turbo makes enough boost and flows enough air to produce a vacuum on the suction side of the turbo, the regulator valve diaphragm sucks closed and blocks off the larger port.

This leaves only the smaller port open to flow crankcase breather fumes.

Here are two diagrams showing the valve in operation under low boost and low intact tract flow with both the 0.180 and the 0.400 ports open, the second diagram shows the operation under high boost, high intact tract flow, high intact tract vacuum conditions when the internal diaphragm is pulled against the 0.400 opening to close it off, leaving only the smaller 0.180 port open for reduced flow.

Diagrams courtesy of Audi of America

NOTE: On the 1991 200TQ 20V there is also a flame arrestor (spiral wire brush) inside this breather hose on the end near the black flow regulating valve, so be sure to remove this flame arrestor and install it in the replacement hose. This flame arrestor prevents a back fire in the intake boot from traveling into the engine crankcase. I have not verified if the S4 uses this same flame arrestor.

This 1991 200TQ 20V breather hose also has a Tee in the center of the hose, which connects to the intake manifold via a flow regulating one way check valve and a molded rubber hose. The check valve will close under positive manifold "boost" pressure.

On the 1991 200TQ 20V the carbon canister solenoid valve also connects at this tee location. The carbon canister solenoid allows gasoline fumes from the fuel tank that are stored in the carbon canister, to flow into the intake manifold when the engine is running.

The carbon canister solenoid valve is closed when the engine is off to prevent fuel tank fumes from entering the engine and causing a rich mixture when starting. If the carbon canister solenoid or electrical connection should fail, this valve also has an internal diaphragm which will open under engine vacuum to allow fuel tank fumes to flow into the engine. It is very difficult to see this connection to the intake manifold hose, as there is very little room behind the engine.

NOTE: On the 1991 200TQ 20V if you are working around the back of the engine and replacing this breather hose, make sure you mark and identify any hoses or electrical connectors before removing them. In one instance, I ran across a car that had the electrical connectors from the heater valve temp sensor, the carbon canister valve, and the engine coolant temp sensor mixed up and connected to the wrong item.

The color coding on the connector, may not match up with the item it plugs into. The Bentley manual shows that the Engine Coolant temp sensor connector should have two wires, one Gray with brown stripe, one Green with black stripe. The Carbon Canister solenoid connector has two wires, a Red colored wire and a White with red stripe wire.

The heater valve coolant temp sensor electrical connector should have two wires, a Yellow and red stripe wire and a Brown with black stripe wire. The two small idle stabilizer hoses should be removed and checked also as they can get blown off from the pressure. Any of the smaller hoses connected from the intake manifold to the heater/AC system should be checked for cracks or leaks.

Are you using the correct Spark Plugs?

The 20V Turbo engines as found in the 1991 200TQ and in the later 1992-95 S4/S6 should have installed the Original Equipment F5DPOR BOSCH PLATIN Electrode Spark Plugs.

Be aware, that they are more expensive than normal spark plugs. These F5DPOR plugs have a single thick platinum electrode.

These thick electrode Bosch PLATIN plugs should not be confused with the cheaper Bosch platinum spark plugs that have a thin center electrode. There are Bosch 3 electrode copper core plugs (FR5 DTC) that cross reference to the F5DPOR, but I don't recommend using these 3 electrode copper core plugs, use the correct single thick electrode platinum F5DPOR plugs.

It is also a good idea to check the torque of the spark plugs, every 5k miles if you are running high boost levels, as many have found the plugs to come loose on the 20V Turbo engines.

New Bosch F5DPOR plugs are spec'd to be tightened to 22 lb-ft, used plugs will normally require slightly more torque when re-installing them.

It is also a good idea to replace the valve cover gasket on the 20V Turbo engines, if you have higher mileage on your engine, and still have the original gasket.

The center valve cover gaskets around the spark plug holes can leak and mess up the plug area and the spark plug wire ends.

Ignition Wires causing misfire?
Distributor Cap shorting out to black shield cover?

For the 3B engine, the Audi OE ignition wires have a stranded copper wire and use resistive spark plug connectors that each have ~5k ohms. The other end of the ignition wire that plugs into the distributor cap has a resistive connector end with approximately 1k ohms.

In many cases the spark plug ends can have too much resistance and will cause a misfire under load, in some cases the resistance measurement will test OK, but you will still get a misfire due to the ignition wires or connectors.

On high mileage engines with the original ignition wire set, the spark plug wires and connectors can arc to the valve cover under high load/high boost.

I have also found in rare cases that the distributor cap can arc internally to the inside of the outer black plastic shield which can cause strange running conditions and misfires that are very difficult to locate.

The distributor cap below was arcing from the coil wire center post to the black cap shield.

It came from a 1991 200TQ 20V that exhibited hard starting intermittently, would stall at idle at random times, and would cutout under full throttle conditions that felt like the fuel pump quit. The ECU would store false fault codes for the RPM and Reference Sensors, as well as a false fault code for the Hall Effect Sensor in the distributor. The arcing area was on the back side of the cap and was not visible with it installed on the distributor.


1992-97 Audi S4/S6 Five Cylinder 20V turbo engine:
The coil leads that connect from the coil to the spark plug also have internal resistance (~5k ohms) which can fail and cause a misfire under heavy load even if the resistance measures OK. Additional information below covering other ignition problems.

Ignition misfire on S4/S6 with coil over direct ignition system

Are you using the correct Distributor Rotor for the 3B 20V Turbo Engine in the 1991 200TQ ?

The 3B designated 20V Turbo engine in the 1991 200TQ used a unique Distributor Rotor with a narrow tip that can be difficult to find in the US. This narrow tip rotor is required to provide the precise ignition timing at the higher RPM's and to prevent any ignition cross firing inside the distributor cap. Some people have reported engine damage (bent rod) that may have been caused when the incorrect cylinder was fired under very high boost.

The Bosch parts catalog shows the INCORRECT distributor rotor
(Bosch 04170 or the 1 234 332 350 R1) for this 20V Turbo application.

This Bosch 04170 (1 234 332 350 R1) rotor has a wider electrode tip measuring 0.710 inches (~17-18mm).

The Audi part number shown for this INCORRECT rotor is 052 905 225C

The correct Bosch Distributor Rotor with the narrow tip for the 3B engine, has the last 3 digits as -414.

The correct narrow electrode tip rotor measures only 0.432 inches, (~11.0mm).

The Bosch part number may also have the designation "R1" at the end, which indicates the rotor has a 1k ohm resistive element from the center to the tip.

The original 3B distributor rotor with narrow tip. The other important thing to note about this rotor, is that it is glued onto the distributor shaft, and it needs to be broken or crushed when removing it from the shaft using a pair of channel lock pliers. You can also use a dremel cutting wheel to cut off the old rotor.

Be careful not to damage the shaft when crushing this rotor and removing the left over pieces. You need to thoroughly clean the distributor shaft with some sand paper and some non residue cleaning solvent, (loctite primer solution is also recommended) and then use Loctite 640 (or equivalent) to secure the new rotor in place.

The Loctite needs at least 4 hours to dry before you can start the engine. This operation is not easy given the restricted access to the distributor on the 3B 20V Engine. You can remove the intake manifold to allow easier access to the distributor.

NOTE: Even though it is possible to pry off the anti-tamper plate from over the distributor hold down nut with the intake manifold installed, I recommend taking the extra time and remove the intake manifold to allow easy access to the ignition distributor.

With the intake manifold removed, you may want to replace the coolant pipe O-Rings, seal and Turbo coolant hose while you have the intake manifold removed. See section below for details. When you unbolt the intake manifold, you can leave the fuel lines attached to the intake fuel rail, and just use some heavy wire to tie up the intake manifold out of the way as shown in the Bentley repair manual.

A long (over 8 inch) 6mm allen wrench tool is usually required to remove the intake manifold bolts. (VAG1669 or equivalent). You can also use a long 1/4 inch drive extension, with a 6mm socket and a 6mm allen wrench socket shaft in combination to access these intake manifold bolts but this is a little tricky.

The intake manifold gasket should be replaced and the bolts should be torqued to 16 ft-lbs upon reassembly, start in the middle and work your way outward when torquing them down. Once you have the intake manifold tied up out of the way, you can cut off the anti tamper cover (a Dremel cutting wheel works here as well).

Before you loosen the distributor hold down nut, you should first set the engine to be at TDC with the flywheel O mark lined up, and the distributor rotor pointed at the line mark on the rim of the distributor. Once you get the engine rotated to the correct position, you can remove the hold down clamp and pull out the distributor. Removing the distributor gives you better access to the distributor so you can either crush the old rotor with some pliers, or cut the old rotor off "carefully" using a dremel cutting wheel.

As mentioned, you should remove all traces of the old rotor with some 200-300 grit sandpaper, and use some non-residue solvent (loctite primer solution is also recommended) to clean the distributor shaft before you apply some Loctite 640 to the inside area on the new distributor rotor. Insert the rotor over the distributor shaft with the key way lined up. Allow to dry for at least 4 hours before starting the engine.

The Bentley Repair manual also indicates that you should use the special tool #3233 to accurately line up the distributor rotor tip when setting the basic distributor position in relation to the flywheel O TDC mark, and the cam gear dot mark.

The Bentley also states that you should turn the distributor body counter-clockwise when you line up the rotor with the 3233 tool and then hold the distributor body with your hand while you tighten down the hold down nut. NOTE: The R1 in the distributor rotor part number refers to the fact that this rotor has a 1K ohm resistor between the center electrode and the outer tip electrode.

This rotor resistor can sometimes burn up after many years and cause a no start, and prevent the engine from running. You may also want to replace the valve cover gasket, the main breather hose, any vacuum hoses and other hard to reach coolant hoses and coolant temperature sensors while the intake manifold is moved out of the way.

See section below for replacing the Coolant Pipe O-Rings, seal and turbo coolant hose while you have the intake manifold off.

Coolant Pipe O-Rings, Seal and Turbo coolant hose replacement

Knock Sensor Intermittent? Hesitation? Low Power? Check Engine light On?

If your 1991 200TQ 20V is running poorly or the "Check Engine" light has come on, it is possible that the ECU is detecting a fault in the knock sensor system caused by a loose, defective or intermittent knock sensor. There may also be a problem with the knock sensor wiring or connectors.

Go to 20V Engine Knock Sensors section for details on the 20V Turbo Engine knock sensors.

The newer style "replacement" knock sensors have gold plated terminals and the wiring harness connector terminal pins should be replaced with these gold plated versions as well.
Some of the 1991 200T/Q's have the "Check Engine" light bulb removed from the dash, so if the car is having a problem with one or both of the knock sensors, you may not know it.

The "Check Engine" light should come on with the ignition key turned on to the first position.
Air Temp Sensor

The air temp sensor that the ECU monitors to regulate boost and ignition timing can have poor external connections or can have intermittent connections on the small thermistor at the tip of the sensor. This sensor is located in the intake manifold near the throttle valve.

The wire terminals are normally soldered or spot welded to the air temp sensor. Factory replacement air temp sensors use a separate connector with gold plated terminals instead of the soldered/welded terminals on the original sensor.

Go to the 20V Air Temperature sensor page for details on checking the operation of this air temp sensor.

O2 Sensor Replacement

The O2 sensor should be replaced if it has over 60k miles on it, or if you have a hesitation, lean surging during light throttle conditions, poor cold running problems during the first 30-60 seconds of engine warm up, poor gas mileage or emissions related failures.

Older O2 sensors can get contaminated and will respond too slowly to correctly adjust the mixture and can cause poor running when transitioning from light throttle to full throttle conditions.
See these sections for details on testing and /replacing the O2 sensor.
Oxygen (O2) Sensor, Open and Closed Loop Operation

Oxygen (O2) Sensor, Testing

Bosch Replacement 3 Wire Oxygen Sensor

Fuel Filter clogged or restricted?

Fuel contamination?

A clogged or restricted fuel filter can cause cutting out under high load and high boost in 3rd/4th/5th gear that often may feel like an ignition misfire.

It is a good idea to change out the filter every 30k miles or every 2 years.

The filters are normally designed to absorb any moisture in the fuel which can cause the filter media to swell and restrict fuel flow.

The ethanol based fuel can make this issue worse. After removing the filter, if you can't easily blow through the filter, it needs replacement.

If I suspect fuel contamination with excessive water or dirt etc, I like to remove the filter and "carefully" blow the remaining fuel out of the filter into a clear glass bottle to check for any crud/water.

Fuel Pressure Regulator Diaphragm Leaking, Running rich?

Regulator Vacuum/Boost hose leaking or disconnected,
Poor running, cutout under full load?

In some cases the fuel pressure regulator diaphragm is ruptured and will leak fuel into the vacuum/boost port and get sucked into the engine at the other end of the hose where it attaches to the intake manifold. Check the vacuum port for any fuel after removing the vacuum/boost rubber hose line.

Also verify the vacuum/boost hose from the intake manifold to the fuel pressure regulator vacuum/boost port is not leaking.

If the fuel pressure regulator does not get good engine vacuum at idle, this will cause the fuel pressure to be increased and this would cause rich running at idle.

The regulator also requires boost pressure acting on the diaphragm to ensure the fuel pressure is increased above manifold boost pressure to maintain the fuel pressure drop across the injector.

Insufficient fuel pressure under high load, high boost can cause cutting out in 3rd/4th/5th gear.

The S4/S6 incorporates a screen inside this pressure regulator which could get clogged with extremely contaminated fuel.

Multi-Function Temp. Sensor-Defective?

One additional sensor to check when having low boost problems (1991 200Tq 20V Only) , is the Multi-function Temperature sensor.

This sensor is mounted underneath the intake manifold, on the coolant pipe on the 20V Turbo Engine.

This Multi-function temp sensor uses a 4 terminal connector which is normally is covered with a protective rubber boot. The newer style replacement Sensors will have only 3 terminals as they simplified the internal design of this sensor and eliminated the +12V supply to this sensor.

Go to 20V Engine Multifunction Sensor section for more details on this sensor.
Audi Parts

Coolant Temperature Sensor:

The Engine Control Unit uses a coolant temp sensor that should be tested if cold or hot start problems or problems during engine warmup are occurring.

Go to 20V Engine Coolant Temperature Sensor section for details on checking this sensor.

The engine must warm up quickly and run hot enough for correct boost control and timing control and best performance. To ensure correct operation and for the highest fuel mileage, only use the 87 Celsius Thermostat

See cooling system section below.

Check ECU Connector Terminals for Corrosion

If you are having some running or boost problems, you may want to inspect the ECU Connector terminals for any subtle corrosion on the tin plated terminals and use some electronic contact/terminal cleaner on them, along with some contact enhancer.

The plastic bag cover over the connector should be installed correctly to avoid any water or moisture from dripping down onto the connector and wiring. Sometimes just removing the large ECU Connector will cure some problems as the action of removing and installing the large connector wipes clean the connector terminals.

Check Engine Ground Cable and Intake Manifold Ground wires

Throttle Cable Adjustment

It is a good idea to check the throttle cable adjustment to ensure the throttle is being opened all the way when the gas pedal is pushed all the way down to the floor. With the engine off, have a friend push down on the gas pedal inside the car, while you check the throttle lever under the hood. The throttle lever should be pulled all the way open with the gas pedal pushed down all the way to the floor

Idle Switch or Throttle Potentiometer Defective?

The Throttle switch/Potentiometer has 2 functions, (1) the idle switch tells the ECU that the throttle has closed so it can perform deceleration fuel cutoff and correct idle speed regulation.

(2) The Potentiometer (variable resistance) tells the ECU both the throttle position and the rate the throttle is opened and closed.

This resistance measurement is used with the ECU voltage supply to provide a varying output voltage the ECU reads to set the correct amount of boost produced.

A defective Throttle Potentiometer may prevent the correct operation of the Waste Gate (WG) solenoid system that the ECU uses to adjust the turbo boost.

Go to 20V Throttle Switch/Potentiometer page for details on checking the Throttle Potentiometer.

Note: Using a stiffer Waste Gate spring is a great idea on the MC engines and possibly on some 20V engines with non-stock Turbos as it will improve the rate at which boost is produced, but...... it can mask problems with the ECU system components (Throttle switch/throttle potentiometer etc.) as the boost will be mostly controlled by the stiffer Waste Gate Spring.

Check basic engine timing, flywheel, camshaft and distributor rotor position

You should check the basic engine timing, the flywheel should be placed at TDC with the O mark lined up in the transmission window, the camshaft gear line should be set at the top mark on the valve cover, and the distributor rotor should be set to the #1 cylinder line on the rim of the distributor.

I have run into a few 20V engines that had loose front crank damper pulley bolts, which allowed the crank timing gear to move side to side and shear off the keyway which altered the crank/cam timing. It is likely that the previous timing belt repair work was done incorrectly, and the mechanic did not torque the crank pulley bolt enough.

The line on the dual mass flywheel in the AAN S4/S6 20V turbo engines can be tough to see through the hole in the transmission housing when setting the engine to TDC, it helps to put a white paint mark on this line. In some cases on the 3B 20V Turbo engines, the timing belt will stretch over time, and will change the cam/distributor timing enough to cause problems for the ECU when it processes the distributor hall effect signal to set timing and boost.

The Bentley manual has the details on setting these 3B distributors correctly, normally this is done when the timing belt, or distributor is replaced or if the camshaft is removed. A special tool #3233 is normally used to line up the distributor rotor with the line on the distributor body, which can help as it is difficult to see the distributor behind the intake manifold.

See photo below

If you don't have the 3233 tool, a small mirror can also be used to correctly line up the distributor rotor and the line the rim of the distributor.

See section below on using the correct distributor rotor for these 20V "3B" designated engines.

Altitude Sensor ok?

Idle Stabilizer Valve (ISV)

The idle stabilizer valve (ISV) can also stick from oil/crude built up inside and this can cause strange high idle conditions.

Cleaning the ISV with carb cleaner may help.

With the ISV removed from the engine, but with the electrical connector on, I like to run the ECU Output Tests which cycle this ISV open and closed while you spray the cleaner inside.

The hoses on the ISV valve to the intake manifold can blow off under boost, or can be cracked and cause a loss in boost pressure. This engine uses the ECU to control the idle stabilizer valve, which is a rotary type valve.

Go to 20V Idle Stabilizer System section for details on checking the operation of this rotary valve

Idle Speed/Mixture Adjustment/Check:

NOTE: The Idle speed and Idle mixture on the 20V engine are not adjustable.

The 20V Motronic Engine Management system has the capability to adjust the fuel mixture over time (adaptive system) so periodic adjustments have been eliminated [1].
Fuel Injectors

Leaky injectors can increase the hot/cold starting times but the 20V engines are not as susceptible to this leaky injector problem as the previous 10V Engines. Some high mileage vehicles or vehicles that are driven over repeated short distances without proper warm up may exhibit fuel spray problems. Dirty/clogged injectors with poor spray patterns can cause poor low end performance and uneven performance when cold. There are several companies that offer fuel injector cleaning and balancing etc.

Excessive Intake valve carbon deposits can also cause cold running problems as the carbon acts like a sponge, soaking up the fuel at first. With severe carbon buildup some repair shops can use a walnut shell blasting device to clean off the carbon buildup. Lighter buildup can be removed with the use of a fuel additive or a fuel with Techron.

Coolant Pipe O-Rings, Seal and Turbo coolant hose replacement

Cooling System checks

The engines cooling system needs to be working correctly before you decide to up the boost on your car.

All the coolant hoses should be checked/replaced and the cooling system should be flushed out and refilled with the correct mix of coolant. When you have the upper radiator hose removed, check the condition of the plastic upper radiator hose fitting, as these plastic fittings can crystallize and break off while driving.

On the 5000TQ and the 200TQ, the plastic heater valve located at the back of the engine near the exhaust waste gate should be replaced if it has high mileage, they have a bad habit of getting brittle and breaking, causing a loss of coolant and some rapid overheating.

The S4/S6 have this heater valve located away from the heat of the engine underneath the plastic cowling near the windshield, so they may not need to be replaced as often.

The S4/S6 do have a plastic Tee fitting that has the Climate Control temperature sensor, which can get brittle over time and break that will cause you to lose most of your coolant while driving.

This Tee fitting should be replaced on high mileage cars to avoid a sudden loss of coolant. We keep these in stock , if you need one.

The Heater hose fitting at the back of the engine block is also plastic and can crack internally which will cause a coolant leak, best bet is to replace both of these plastic fittings.
The thermostat should be replaced with the factory specified 87 degree Celsius version if you still have the original one installed, or if it is several years old. Often times a malfunctioning thermostat will cause engine running problems, and poor gas mileage if the engine is running too cool.

You may also want to replace the short piece of coolant hose located underneath the intake manifold, as this hose can blow and make your life miserable if you have to change it when the engine is hot.

Audi sells the hose in bulk lengths, or you can look for similar sized coolant hose from an aftermarket parts supplier, Gates etc. I don't recommend using fuel line or other hose not designed for high temperatures. The after run cooling fan and turbo pump should be checked for correct operation.

Go to the Cooling system page in the Troubleshooting section for additional details.

Exhaust System Issues

You should have the exhaust back pressure checked by a muffler shop if you have low boost conditions and suspect a plugged catalytic converter or muffler/exhaust system. They can use the pre-catalytic converter sniffer pipe bolted to the intake manifold to do this, or they can drill a small hole in the exhaust pipe to insert the pressure gauge pipe.

A low pressure (0-10 psi) fuel pump pressure gauge can be used for this. Typically with the car idling in neutral, as you slowly rev the engine to 4000 RPM and hold it at 4000 RPM, the exhaust back pressure should not increase above 1-2 psi. If you run a long hose from the sniffer pipe to a gauge inside the car, an assistant can check the exhaust back pressure while you drive the car.

Test Example: When the car is driven and under load in 3rd gear at WOT and near redline, it is not unusual to see over 4-7psi of back pressure when running 1.8 bar of boost pressure. The older catalytic converters can create some of this back pressure and may need to be replaced as the car gets over 75K miles.

The RS2 type exhaust manifold is recommended for improving the exhaust flow out of the head and rumor has it, to prevent burned exhaust valves when more extensive ECU and Turbocharger mods are performed. As mentioned, the catalytic converter and muffler system can get plugged or restricted over the life of a car which may cause poor running with low power.

Catalytic Converter meltdown can occur if extremely rich mixture levels occur in the combustion chamber which are not thoroughly burned which allows high levels of raw gasoline (Hydrocarbons) to pass into the catalytic converter. If this occurs under engine load conditions and is followed by normal lean running or idling the catalytic converter will get very hot and damage may occur. Ignition misfires can also dump raw fuel or hydrocarbons (HC) into the catalytic converter and this will also cause meltdown of the ceramic honeycomb material if the engine is driven under load.

If you ever need a reminder to let your engine run after a high boost run, just open up your hood at night and see the red hot exhaust manifold and turbo! Even with the water cooled turbo you should let the car idle for several minutes.

It may take 10-15 minutes of normal low boost driving before the exhaust manifold/turbo cool down. It is a good idea to avoid high boost initially after starting a cold engine until the oil temp gets up to 60C (this is recommended in the factory operators manual).
Drive Train Issues

Driving with more boost/horsepower places more stress on the transmission, clutch, suspension system and tires. The Audi I5 turbo engines are very robust motors but running Higher Boost levels obviously places some additional stress on the internal engine components.

The two hydraulic engine mounts and the rear transmission mounts should be checked, the exhaust side (passenger side) engine mount is subjected to a lot of heat. A defective hydraulic engine mount on this side tends to place more stress on the exhaust manifold/system.

Of course the tires, CV joints, wheel bearings, shocks, sub frame bushings and other suspension bushings/parts subject to wear and tear should be replaced where necessary. Having more power will make the car a handful to drive if you have a worn out suspension as you will discover in short order.

Better to fix it now than to crash and burn it later.

References: [1] Audi of America, Technical service training publication:
"The New 20V Turbo Engine for the Audi 200 Quattro-publication
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