Overview
This document contains information on:
General
Function requirements
Function
General
Escaping fuel vapors are retained by the purge system, stored and supplied to the combustion process. The purge system is checked at regular intervals for leaks. The regular checks ensure the operational readiness of the purge system and compliance with the legal emission limit values. The leak test is a measure for determining and classifying leaks.
Function requirements
Engine management ON (circuit 87M)
Combustion engine IDLING
Vehicle stationary
Drive position D or R engaged
Blocking time of approx. 3 min after engine start has expired or self-adjustment of mixture formation has taken place
Lambda control released
Coolant temperature at combustion engine start < 100 °C
Ambient temperature between 4 °C and 35 °C
Air pressure < 780 hPa, i.e. no check takes place above an altitude of approx. 2500 m
Low loading of activated charcoal canister
Fuel level in fuel tank at least 30 %
No fault in the activated charcoal canister shutoff valve
No fault on fuel level indicator fill level sensor
No fault in the purging switchover valve
No fault in fuel system control unit
Battery voltage > 11 V
Tank cap closed
Function
Leak test
The leak test takes place in following three stages:
1 Gross leak ( Ø 2.5 mm)
2 Fine leak ( Ø 1.0 mm)
3 Very fine leak ( Ø0.5 mm)
Leak test for a gross leak
During the gross leak test, the purge control valve opens. The vacuum generated by the combustion engine in the intake manifold reaches the fuel tank. A vacuum of approx. - 6 mbar is thus generated in the fuel tank within 12 s. A pressure sensor is located in the fuel tank for the pressure measurement.
Where a vacuum of approx. - 6 mbar is reached in the fuel tank, the next stage involving the leak test for fine leaks is started.
A gross leak exists if no vacuum is generated in the fuel tank (e. g. fuel filler cap open, purge line defective). The leak test is canceled and the fault "gross leak" is saved.
Leak test for a fine leak
During the leak test for fine leaks, a vacuum of approx. - 6 mbar is again generated in the fuel tank and the system is made airtight. The vacuum is then measured for approx. 30 s by the pressure sensor in the fuel tank. The pressure reduction should be in the specified range between 0.3 mbar/s and 0.5 mbar/s, depending on the fuel level.
If the measured pressure lies in the specified range of 0.3 mbar/s to 0.5 mbar/s, the next stage involving the leak test for very fine leaks is started.
A fine leak exists if the vacuum drops more quickly that the values specified in the specified range. The leak test is canceled and the fault "fine leak" is saved.
Leak test for a very fine leak
If no fine leak was detected, the purging is enabled for a few seconds and a vacuum of approx. - 6 mbar is then built up again. In a closed system, the vacuum may not drop faster than 0.10 mbar/s to 0.15 mbar/s depending on the fuel level. If the vacuum decreases more quickly, the fault "very fine leak" is saved.
Detected faults are saved by the combustion engine control unit and transmitted to the instrument cluster control unit. The message "Engine diagnosis via the indicator lamp" is shown in the instrument cluster control unit.