Fuel Tank Venting on the Audi 80

The fuel tank vent line is connected directly behind the filler neck. The vent line first leads to the so-called check valve. The check valve closes the line as soon as the vehicle tilts more than 45°. This prevents fuel from leaking through the vent lines in an accident. After passing the air valve, however, the vent line does not lead directly outside but ends in the activated charcoal canister in the right front wheel well. Its purpose is to capture the fuel vapors passing through this line (generated, for example, by heating of the fuel tank contents). When the engine is running, these vapors are drawn back out of the activated charcoal canister under certain engine loads. When this should occur is determined by the load-controlled solenoid valve.

In parallel with this system, the air valve opens during refueling to the fuel tank expansion tank, located in the filler channel (in the right rear wheel well) as a separate container. In this way, as the fuel level rises, fuel vapors can be displaced into it instead of escaping into the atmosphere.

While driving, outside air enters the tank depending on the amount of fuel used, so that no vacuum forms in the tank.