Evaporative Emissions System: Description and Operation

Function Description of EVAP Canister System
Depending upon the air pressure and ambient temperature, fuel vapor will form above the level of fuel in the tank.

The Evaporative Emissions system prevents these HC emissions escaping to the atmosphere.

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

The activated charcoal stores these vapors like a sponge.

When the oxygen sensor control is active when driving (engine warm), the solenoid valve (N80, also called regeneration valve), is activated (pulsed) from 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 activated charcoal filter during the purging procedure (regenerating the activated charcoal). The fuel vapors stored in the activated charcoal and fresh air are pulse-fed for combustion.

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.

No voltage (e.g. wiring open circuit) the solenoid valve is closed and the activated charcoal filter will not be purged.

The vacuum pipe from the throttle valve control part to the activated charcoal filter has an additional by-pass which is connected with the cylinder block breather valve. The non-return valve integrated in the cylinder block breather valve prevents the intake air gassing into the cylinder block during some partial load ranges. This improves the overall cylinder block breathing. The by-pass guarantees the cylinder block breathing.

Functional description of leak diagnosis
The evaporative emissions system (including fuel tank) is equipped with a leak diagnosis, the leak diagnosis will detect whether the system is leaking.

The diagnosis operates by pressurizing the system and should detect leaks where the damage exceeds 1 mm in diameter.

During the diagnosis the fuel system diagnostic pump -V144- builds up a pressure of approx. 30 mbar in the evaporative emissions system. The pump will switch off when the pressure is attained. When the pressure falls to below a certain figure, the pump will switch on again. The On Board Diagnostic (OBD) monitors the time between the switched on periods and if it is too short it will register a fault in the DTC memory.


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.


Observe safety precautions.
Observe rules for cleanliness.