Diagnostic module for tank leaks (US, Canada, Korea)

The leak test of the fuel system is run regularly after stopping the engine. The following processes run in the after-running period of the DME:

• Initial situation

During normal engine operation, the changeover valve in the diagnosis module is in the position "Regeneration". Fuel vapors are stored in the carbon canister and fed into the engine depending on the actuation of the tank vent valve (refer to FUEL TANK VENTILATION ).

• Checking start conditions

  The tank-leak diagnosis is started under the following conditions:

  • Engine off: engine management system in after-run (approx. 25 seconds waiting period)
  • Cold start: coolant temperature less than ambient temperature plus 6.8 °C
  • Duration of the current journey: greater than 10 minutes (totalled over 40 km/h)
  • Fuel tank level: between 7.5 liters and 42.5 liters
  • Ambient temperature: between 1.5 and 38.25 °C
  • Ambient pressure: greater than 730 mbar
  • Filling of activated carbon filter: less than limit value (virtually full)
  • Battery voltage: between 10.9 and 16 Volts

  With a positive result, the tank leak diagnosis is started with a comparison measurement.

• Comparison measurement

  The tank vent valve is always closed after the engine is switched off. The changeover valve of the diagnosis module remains in the position "Regeneration". The electric leak diagnosis pump impels fresh air from the surrounding area through a defined leak of 0.5 mm diameter. The power consumption needed for this is stored as a value. The actual tank leak diagnosis then follows.

• Tank leak diagnosis

  The tank vent valve remains closed. The changeover valve of the diagnosis module switches to the position "Diagnosis". The leak diagnosis pump pumps fresh air from the environment into the fuel tank, slowly raising the internal pressure. At the start of the tank leak diagnosis, the internal pressure corresponds to the ambient pressure. This is why power consumption is low. As the pressure inside the tank increases, the power consumption also increases. The DME evaluates the power consumption of the leak diagnosis pump.

• Evaluation of the pump current

  The DME evaluates the rise in the power consumption within a certain time. If the power consumption exceeds the value stored within this time, the fuel system is considered to be OK. Tank leak diagnosis is ended. If the power consumption does not reach the value stored, the fuel system is considered to be defective. Tank leak diagnosis allows a difference to be made between:

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

  The relevant fault is entered in the DME fault memory. Tank leak diagnosis is then terminated.

• End of the tank-leak diagnosis

  The changeover valve is switched back to the position "Regeneration". The after-running period of the DME is available for other functions.

Fig 1: Fuel Tank Leakage Diagnosis Module Characteristic Curve

Courtesy of BMW OF NORTH AMERICA, INC.