Evaporative Emissions System: Description and Operation

Evaporative Emissions (EVAP) System Diagnostic Leak Test, Function


The EVAP system diagnostic leak test recognizes leaks in the entire fuel system including the fuel tank, Evaporative Emission (EVAP) canister, EVAP canister purge regulator valve -N80- and the EVAP canister purge solenoid valve -N115-.

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 the diagnostic leak test is initiated, the Leak Detection Pump (LDP) increases pressure in the fuel system to 20-30 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 and the reed contact closes.
- First the diagnostic will test whether the reed contact closes within 10 seconds. If it doesn't, the fuel system is sealed.
- If the reed contact closes within 10 seconds, however, the system has a leak. If this is the case, the membrane pump will pump four additional strokes. The Engine Control Module (ECM) measures the time between these tour strokes and closure of the reed contact. Based on this measurement, the ECM recognizes whether the leak is larger than 1 mm in diameter and can distinguish a "small leak" from a "large leak."

Leak testing is initiated 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 if the malfunction is recognized for two subsequent starts. Leak detection can also be activated using the VAS5051 tester or VAG1551 scan tool. Testing and Inspection