Function of EVAP System

Function Of EVAP System (Engine Codes AVH, BEV)

Depending on air pressure and ambient temperature, fuel vapor will form above the level of fuel in the tank.

The EVAP system prevents these hydrocarbon emissions from escaping into the atmosphere.

The fuel vapors from the highest point in the pass through the gravity valve (which closes at an angle of 45 degrees ) and through a pressure retention valve (in limited quantities) to the EVAP filter.

The activated charcoal in the EVAP canister stores these vapors like a sponge.

When the oxygen sensor control is active during driving (engine warm), the EVAP canister purge regulator valve -N80- is activated (pulsed) from engine control module depending upon load and engine speed. The opening period is dependent on the input signals.

The intake manifold vacuum draws fresh air through the breather 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 is fed for combustion in dosed quantities via the EVAP canister purge regulator valve -N80-.

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

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

The vacuum line from the throttle valve control module to the EVAP canister has an additional by-pass 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 by-pass guarantees the crankcase breathing.

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

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

During the diagnosis the fuel system Leak Detection Pump (LDP) -144- 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 On Board Diagnostic (OBD) monitors the switching periods and sets a fault in the Diagnostic Trouble Code (DTC) memory when the period is too short.

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







- Assembly tool VAS 5024 or hose clip pliers V.A.G 1921 are recommended for installing spring-type clips.