Volkswagen Front Wheel Drive 1974-1989 Repair Guide

Evaporative Canister

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SERVICING



See Figures 1, 2 and 3

All models 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. On most models, a control valve is used to prevent these vapors from entering the intake manifold when the engine is not running. When the engine is running, these vapors are carried to the intake manifold by allowing fresh air into the bottom of the canister. On vehicles with CIS-E fuel injection, the carbon canister is isolated from the intake manifold by a solenoid valve and a frequency valve. The solenoid valve is ON or open whenever the engine is running. The frequency valve is cycled open and closed by the Motronic control unit to control flow rate depending on coolant temperature and engine speed/load conditions. The canister is usually located in one of the front inner fenders.



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Fig. Fig. 1: Most evaporative systems are equipped with a vacuum operated control valve



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Fig. Fig. 2: On vehicles with CIS-E fuel injection, the carbon canister is isolated from the intake manifold by a solenoid valve and a frequency valve

Other components of the system include an unvented fuel filler cap, fuel tank expansion chamber and one or more check valves to prevent liquid fuel from entering the canister. On most models, the purge hose must be disconnected at the canister when checking or adjusting air/fuel mixture. Make sure the orifice is still connected to the hose to the intake manifold.

This system does not require any service under normal conditions other than to check for leaks. Check the hoses visually for cracks, breaks, and disconnections. 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 for fuel system use. These are usually available from your local automotive supply store.



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Fig. Fig. 3: The canister is usually located in one of the front inner fenders

 
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