VW Passat & Audi A4 1990-2000

Evaporative Canister



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Fig. Evaporative canister frequency valve and control valve system used 2.0L 16v engines

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Fig. Evaporative system used on early VW models

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Fig. The EVAP purge valve is mounted on the air filter housing on 1.8L models

All vehicles with gasoline engines are equipped with some form of fuel vapor control device. The evaporative emission control system prevents the escape of raw fuel vapors (unburned hydrocarbons, or HC) into the atmosphere. A carbon (charcoal) canister is used to store fuel tank vapors that accumulate when the engine is not running. When the engine is running, the vapors in the canister are carried to the intake manifold by allowing fresh air into the bottom of the canister.

Other components of the system include a non-vented fuel filler cap, fuel tank expansion chamber and one or more check valves to prevent liquid fuel from entering the canister.

Early models use a vacuum-operated valve to vent the fuel vapors into the intake tract; later models use an electric valve actuated by the Electronic Control Module (ECM).

The evaporative canister is located in the engine compartment.

The evaporative control system does not require any service under normal conditions other than to check for leaks. Check the hoses visually for cracks, breaks, etc. Also check the seal on the gas tank filler cap. Replace the cap if the is split. If any hoses are in need of replacement, use only hoses marked EVAP, available from your local automotive supply store. If you suspect a problem with the purge valve or any other component, see for information on testing the system.

On 1996 and later vehicles, if a problem occurs with the EVAP system, the Check Engine light will be activated.