Evaporative Emissions (EVAP) System Diagnostic Leak Test

Evaporative Emissions (EVAP) System Diagnostic Leak Test

Function
The Evaporative Emissions (EVAP) system, diagnostic leak test recognizes leaks in the entire fuel system including the fuel tank, EVAP canister and Evaporative Emission (EVAP) Canister Purge Regulator Valve N80.

On the mechanical side, the leak detection system consists mainly of a vacuum-driven diaphragm pump, with a reed contact on the actuation rod of the pump diaphragm. The pump is supplied with vacuum via the engine intake.

- When the diagnostic leak test is activated, the Leak Detection Pump (LDP) increases pressure in the fuel system to 20 - 30 mbar. The pump membrane is then expanded so that the reed contact at the operating rod remains in the "open" position.
- The diagnostic will test whether the reed contact remains open within 10 seconds. This indicates the system is sealed.
- If pressure falls (indicating a leak), the diaphragm moves back and the reed contact closes.
- If reed contact closes within 10 seconds of leak detection time, a further test is performed: In this case the diaphragm pump, pumps 4 times again. The engine control module (ECM) measures the time for each of the 4 pumps until the reed contact is closed again. From that, the control module can recognize a "small leak" (greater than 1 mm in diameter) or a "large leak".

Leak diagnostic is activated automatically shortly following every engine start. If a malfunction is determined, an entry is made to the DTC memory. The Malfunction Indicator Lamp (MIL) in the instrument cluster lights up if the malfunction is recognized for two subsequent starts. Leak diagnosis can also be activated using the vehicle diagnosis, test and information system VAS 5051 A.

Automatic leak detection system
With the engine running, the engine control module may perform a leak diagnosis at specific intervals.

It can be determined if an automatic leak diagnosis was recently performed by using the vehicle diagnosis, test and information system VAS 5051 A in measuring value block 071.