Evaporative Emissions (EVAP) System

Evaporative Emissions (EVAP) system
Function
Depending on air pressure and ambient temperature, fuel vapor will form above the level of fuel in the tank.
The EVAP system prevents these HC emissions from escaping into the atmosphere.

In limited quantities, fuel vapors located at the highest point in the tank pass through a gravity valve (which closes at an angle of 45°) and through a pressure retention valve into the EVAP canister.

The activated charcoal in the canister stores these vapors like a sponge.
When the oxygen sensor control is active when driving (engine warm), the EVAP canister purge regulator valve -N80- is activated (pulsed) from the engine control module depending upon load and engine speed. The opening period is dependent on the input signals.

Intake manifold vacuum draws fresh air through the vent opening on the underside of the EVAP canister during the purging procedure (regenerating the activated charcoal). The fuel vapors stored in the activated charcoal and fresh air are fed for combustion in dosed quantities.

The pressure retention valve prevents fuel vapors from being drawn from the tank when the solenoid valve is open and an intake manifold vacuum is present. It therefore ensures that the evacuation of the EVAP canister has priority.

When no voltage is applied (e.g. wiring open circuit), the solenoid valve is closed. The EVAP canister will not be purged.

The vacuum line from the throttle valve control module to the EVAP canister has an additional bypass which is connected with the crankcase breather valve. The non-return valve integrated in the crankcase breather valve prevents the intake air gassing into the crankcase during some partial load ranges. This improves the overall crankcase breathing. The bypass guarantees the crankcase breathing.

Functional description of leak diagnosis
The EVAP system (including the fuel tank) is equipped with a leak diagnosis. This diagnosis system detects leaks in breather system.

This diagnosis is based on the pressure principle and should detect leaks with a circumference larger than 1 mm.

During the diagnosis the Leak Detection Pump (LDP) -V144- creates a pressure of 30 mbar in the EVAP system. When the pressure is attained the pump switches off. If the pressure drops below a predetermined value the pump switches on again. The self-diagnosis monitors the switching periods and sets a fault in the fault memory when the period is too short.

Observe safety precautions.
Observe rules for cleanliness.
Refer to EVAP system components, servicing.
Refer to EVAP system, assembly overview.

Note:
^ Hose connections are secured with either spring-type or clamp-type clips.
^ Always replace clamp-type clips with spring type clips.





^ Always replace clamp-type clips with spring-type clips. Assembly tool VAS5024 or hose clip pliers VAG1921 are recommended for installing spring-type clips.