Evaporative Emissions System: Description and Operation

Function of leak detection system
The Leak Detection system can recognize leaks in the entire fuel system, including the main fuel tank, the EVAP canister and EVAP canister purge regulator valve -N80.

On the mechanical side, the Leak Detection system consists of a vacuum-operated diaphragm pump (LDP pump) with a reed contact on the pump diaphragm rod. The pump receives vacuum from the intake manifold.
- If the Leak Detection system is triggered; the diaphragm pump pressurizes the fuel system between 20 and 30 mbar.
- If specified pressure is reached, the pump diaphragm remains extended and the reed contact remains in the "open" position.
- If the pressure drops (i.e. there is a leak), the diaphragm moves back and the reed contact closes.
- During diagnosis, first there is a measurement of whether the reed contact has closed within 10 seconds. If this does NOT occur, the tank system is then sealed.
- If; however, the reed contact closes within 10 seconds, there is a leak. Then, the pump continues to cycle for four more strokes. The ECM measures the time for each of these four strokes until the next time the reed contact closes. In this fashion, the ECM can differentiate between "fine" and "large" leaks.
- Leak detection is automatically triggered, shortly after each engine start. If there is an incorrect measurement, a malfunction is stored in DTC memory.
- A warning will appear on the instrument panel if the malfunction has occurred during two successive drives. Also, leak detection can also be activated using the VAG1551 Scan Tool See "Leak Detection, Activating and Troubleshooting". Testing and Inspection