Leak detection system

WARNING: This page does not describe the selected car, but rather 15 other vehicles, including the 2008 Audi S4, 2008 Audi RS 4, 2008 Audi A4, 2007 Audi S4, and 2007 Audi RS 4. However, it is still accessible from the selected car via links, so may be relevant.

Function

The leak detection system recognizes leaks in the complete 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, that is equipped with a reed contact on the actuation rod of the pump diaphragm. The pump is supplied with vacuum via the engine intake stroke.

When leak detection is activated, the diaphragm pump pumps the fuel system up to 20 to 30 mbar. The pump membrane is then moved outward, thereby setting the reed contact at the actuation rod to the "open" position.
During leak diagnosis, the reed contact is monitored to check if it remains open within 15 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 15 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" (smaller 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 2 subsequent starts. Leak diagnosis can also be activated using the vehicle diagnosis, test and information system VAS 5051 A → Diagnostic leak test, performing .

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 → Diagnostic leak test, performing .