Evaporative Emissions (EVAP) system diagnostic leak test: Function

WARNING: This page does not describe the selected car, but rather 7 other vehicles, including the 2004 Audi A6, 2003 Audi S6, 2003 Audi A6, 2002 Audi S6, and 2002 Audi A6. However, it is still accessible from the selected car via links, so may be relevant.

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

On the mechanical side, the leak detection system consists primarily of a vacuum activated diaphragm pump with a reed contact on an activating rod at the pump diaphragm. The pump is supplied with vacuum via the engine intake stroke.

When diagnostic leak test is activated, the Leak Detection Pump (LDP) increases pressure in the fuel system to 20 to 30 mbar.
Once the specified pressure is reached, the pump membrane remains expanded so that the reed contact at the operating rod remains in the "open" position.
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 procedure is performed: If this is the case, the membrane pump will pump 4 additional strokes. The Engine Control Module (ECM) measures the time between these 4 strokes and closure of the reed contact. 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 recognized, a DTC entry follows. The Malfunction Indicator Lamp (MIL) in the instrument cluster lights up if the malfunction is recognized for two subsequent starts.