Evaporative Emissions System: Description and Operation

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 the (EVAP) canister purge regulator valve -N80-.
The leak detection system consists of a vacuum activated diaphragm pump with a reed contact on an activating rod at the pump diaphragm. The pump is supplied with vacuum from the engine.
- If the diagnostic leak test is initiated, the Leak Detection Pump (LDP) increases the pressure in the fuel system to 20 - 30 mbar (0.29 - 0.44 psi).
- When the specified pressure is reached,the diaphragm in the pump remains pushed out causing the reed contact at the connecting rod to remain in the "open" position.
- If the pressure falls (which would indicate a leak), the diaphragm moves and the reed contact closes.
- For the diagnosis, the first measurement determines if the reed contact closes within 10 seconds. If the contact does not close, the fuel system does not leak.
- If however the reed contact closes within 10 seconds, the system has a leak and the leak detection pump will pump 4 additional strokes. The Engine Control Module (ECM) measures the time that the pump needs to perform the strokes. The ECM also measures the time for another closing of the reed contact. Based on this measurement, the ECM recognizes whether the leak is larger than 1 mm (0.039 in.) in diameter and it can distinguish between a small leak or a large leak.

The leak test is started automatically shortly after every time the engine is started. If the measurement indicates a leak, the malfunction is stored as a Diagnostic Trouble Code (DTC) in the DTC memory of the ECM. The Malfunction Indicator Lamp (MIL) comes on in the instrument cluster after the malfunction occurs during two subsequent drives. The leak test can also be activated using the VAG 1551 Scan Tool (ST).