TLEV Purge System Function And EVAP Leak Detection Tests

Purge control system monitoring is a function of the ECM. Testing parameters are stored in the module.

Once all of the testing criteria have been met, the ECM will start the test on the purge system. The test is carried out in the following sequence:

- The ECM reads and stores the current fuel tank pressure input value.
- The purge valve and shut-off valve are closed for approx. 8 seconds.
- The ECM reads the pressure build up in the fuel tank. If the pressure build up is below a stored threshold value, the purge valve is switched on for approx. 3 seconds.
- The ECM reads the tank pressure after purging. If the pressure has dropped, the purge function is OK.
- The purge valve is switched OFF again for approx. 8 seconds.
- The MS 41.1 control module reads the tank pressure again to verify pressure build up.
- If the pressure build up is not within a stored value range, the ECM detects this as a leak within the purge system.
- Depending on the degree of pressure build up, the control module can detect between a large leak (fuel filler cap OFF) or a small leak (loose or broken hose).
- Following the test cycle, the shut-off valve is switched OFF, and normal purge operation is resumed.

There are several fault codes relating to the purge system. In addition to fault codes for purge malfunctions or leaks, the MS 41.1 control module also monitors the components for purge system operation and testing. Any detected faults will be stored in the memory and if the criteria for OBD II fault monitoring is met, the Check Engine Lamp will illuminate.

Special tool set, P/N 90 88 6 161150 is used to pressurize the fuel tank and detect leaks in the workshop.