Leak Detection
The leak detection is performed by means of two main phases:
- Tank system over-pressurizing
- Leak magnitude measurement
During the leak detection, the canister purge valve and the canister vent valve (CVV) are closed.
The ECU (Engine Control Module) causes the pump diaphragm to cycle at fixed frequency and for a fixed number of strokes. As air is drawn from outside and pumped into the fuel tank system, the system pressure increases.
Once the tank system over-pressure phase is finished the leak measurement phase starts. The diaphragm stroke is limited by the top of the diaphragm chamber and a position defined by a reed switch level. If the tank pressure drops below a certain value, the LDP will perform a pump stroke in order to maintain the over-pressure in the tank system. Thus the time between pump strokes ("pulse interval") is an indication of the system tightness.
If there is a leak, the cycling time or "pulse interval" stabilizes at a rate, which compares to the leakage loss.
If there is no leak in the system the cycling time or "pulse interval" becomes longer.
The "pulse interval" is measured by the ECU, which determines whether or not the leak exceeds a defined threshold. Several "pulse interval" measurements are carried out to secure the test.