Evaporative Emissions System: Description and Operation

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, 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 its vacuum via the engine intake.

- When diagnostic leak test is activated, the Leak Detection Pump (LDP) increases pressure in the fuel system to 20 to 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. Then the fuel system is leak-free.
- If pressure falls (indicating a leak), the diaphragm moves and the reed contact closes.
- If reed contact closes within 10 seconds of leak detection time, a further test 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 recognizes a "small leak" (greater than 1 mm in diameter) or a "large leak".

Leak diagnostic is activated shortly following every engine start. If a malfunction is recognized, a DTC entry follows. The Malfunction Indicator Lamp (MIL) in the instrument cluster will light up if the malfunction is recognized for two subsequent starts.