Fuel tank leakage diagnosis module

The fuel system leak test is regularly conducted when the engine is switched off. Here, the following processes are executed during the DME run-down period:

• Initial situation

  During normal engine operation, the switchover valve in the diagnosis module is in the "Regeneration" position. Fuel vapors are stored in the activated charcoal filter and fed into the engine depending on the actuation of the fuel evaporation control valve (please refer to "Fuel tank ventilation").

• Starting conditions check

  After the engine is switched off, the conditions necessary to restart are checked:

  • Engine off
  • Battery voltage between 11.5 and 14.5 volts
  • No fault memory entry in the DME regarding diagnosis module for tank leak or tank-ventilation system
  • Fuel level greater than 10 % and less than 90 %

  If the result is positive, tank leakage diagnosis is started with a comparison measurement.

• Comparison measurement

  The fuel evaporation control valve is always closed when the engine is switched off. The switchover valve in the diagnosis module remains in the "Regeneration" position. The electric leakage diagnosis pump impels fresh air from the surrounding area through a defined leak of 0.5 mm diameter. The current draw needed for this is stored as a value. The actual tank leakage diagnosis then follows.

• Tank leakage diagnosis:

  The fuel evaporation control valve remains closed. The switchover valve in the diagnosis module is switched to the "Diagnosis" position. The leakage diagnosis pump impels fresh air from the surrounding area into the tank, causing the interior pressure to slowly increase. At the start of tank leakage diagnosis, the interior pressure is equal to the ambient pressure. The current draw is therefore low. As the pressure inside the tank increases, the current draw also increases. The DME evaluates the current draw of the leakage diagnosis pump.

• Pump current evaluation

  The DME evaluates the increase in the current draw over a certain time.

  If the current draw exceeds the value stored within this time, the fuel system is considered to be OK. Tank leakage diagnosis is ended.

  If the current draw does not reach the value stored, the fuel system is considered to be defective.

  Tank leakage diagnosis allows a difference to be made between:

  • Major leak, e.g. fuel cap missing
  • Minor leak
  • Micro-leak

  The relevant fault is entered in the DME fault memory. Tank leakage diagnosis is then ended.

• End of tank leakage diagnosis:

  The switchover valve is switched back to the "Regeneration" position. The DME run-down period remains available for other functions.

Tank leakage diagnosis can also be started with the BMW diagnosis system. In this case, the processes run as described above.