Evaporative Emissions System: Description and Operation

Evaporative emissions (EVAP) system

Function
Depending upon air pressure and ambient temperature - more or less fuel vapors form in the fuel tank.

The evaporative emissions system prevents these HC-emissions from dispersing into the environment.

Fuel vapors disperse from the highest point of the fuel tank (at the filler neck) through the gravity valve (which closes at an inclination by 45 degrees) into the EVAP canister.

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

When driving with active oxygen sensor regulation (engine warm), the Evaporative Emission (EVAP) canister purge regulator valve -N80- is activated based on load and speed. The open duration is dependent on the input signals.

Without electricity (e.g. open circuit) the EVAP canister purge regulator valve remains closed. The EVAP canister is not purged.

Function description of leak diagnosis
The EVAP system (including fuel tank) is equipped with 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 diagnostic, the Leak Detection Pump (LDP) -V144- builds up a pressure of approx. 30m bar in the EVAP 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. On Board Diagnostic monitors the switch intervals and stores a DTC in DTC memory if the intervals are too short.


NOTE:
- Hose connections are secured with either spring-type or clamp-type clips.
- Always replace clamp-type clips with spring-type clips.
- Fuel hoses in engine compartment must only be secured with spring-type clips. The use of clamp or screw type clips is not permissible.







- The pliers VAS 5024 A are recommended for installing spring-type clips.

Observe safety precautions. Safety Precautions When Working on the Fuel Supply System
Observe rules for cleanliness. [1][2]Technician Safety Information