20V Motronic ECU System,

1991 200TQ and 1992-95 S4/S6 with 20V Turbo Engine

Carbon Canister Frequency Valve

Vapors from the fuel tank are collected in a carbon canister and a frequency valve (solenoid valve) is used to regulate the flow of these fuel vapors into the engine while cruising down the road. This valve is operated by the Motronic Engine Control Unit (ECU) and the duty cycle is varied depending on the engine temperature, load and speed.

When the engine is shut off, the valve is closed to prevent fuel vapors from entering the engine [1]. On the 1991 200TQ 20V this valve is buried behind the back of the engine, below the back of the intake manifold. On the 1992-95 S4/S6 this valve is on the drivers side of the intake manifold near the back, usually blue in color.



NOTE: If power to the frequency valve is interrupted or shut off due to a wiring problem or failure of the solenoid valve while the engine is running, a vacuum connection from the engine intake manifold will open the check valve to allow some fuel tank ventilation.

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

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