Evaporative Emissions System: Description and Operation

EVAP System Leak Detection System, Function

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 (Leak Detection Pump, or LDP) 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 to 20-30 mbar.
- If the 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 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. In this case, the pump continues to cycle for tour more strokes. The Engine Control Module (ECM) measures the time for each of these four strokes until the next time the reed contact closes. The Engine Control Module (ECM) recognizes if the leak is larger than 1 mm diameter and the ECM can differentiate between "small" and "large" leaks.
- Leak detection is automatically triggered shortly after each engine start, EVAP System, Automatic Leak Testing
If there is an incorrect measurement, a malfunction is stored in the DTC memory. A warning will appear on the instrument panel if the malfunction has occurred during two successive drives. Also, the leak detection can also be activated using the Vehicle Diagnostic, Testing and Information System VAS5051 or 1551 scan tool EVAP System, Leak Testing Using VAS5051 or VAG1551