Monitoring function - leak detection

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The evaporative system monitoring permits the detection of leaks in the evaporative system with a diameter of 0.02 inches and up.

By means of a Diagnostic Module Tank Leakage (DM-TL), an electrical actuated pump located at the atmospheric connection of the evaporative canister, a pressure test of the evaporative system is performed in the following order:

During the Reference Leak Measurement, the electrical actuated pump delivers through the reference restriction. The engine-management system measures the pump's electrical current consumption in this section.

Fig 1: Evaporative System Diagram

Courtesy of BMW OF NORTH AMERICA, INC.

During the Leak Measurement, the electrically actuated pump delivers through the charcoal canister into the fuel-tank system. The pressure in the evaporative system may be up to 2.5 kPa depending on the fuel level in the tank. The engine-management system measures the pump's electrical current consumption. A comparison of the currents of the reference leak measurement and the leak measurement is an indication of the leakage in the tank.

Fig 2: Evaporative System Diagram
Courtesy of BMW OF NORTH AMERICA, INC.

0.02 inch diagnosis, very small leak:

P0456

The first step of the diagnosis is the reference measurement, the result of the pump reference current is stored (picture in chapter a). After the solenoid switches, the venting system is pressurized (picture in chapter b). In the small leak measurement the small leak threshold is reached, if the leak is smaller than 0.04 inch and then the small leak measurement phase follows. When the DM-TL current reaches the reference current within the very small leak time, the system is tight (leak smaller than 0.02 inch), otherwise a very small leak between 0.02 - 0.04 inches is detected.

0.04 inch diagnosis, small leak:

P0442

The first step of the diagnosis is also the reference measurement, the result of the pump reference is stored (picture in chapter a). After the solenoid switches, the venting system is pressurized (picture in chapter b). In the small leak phase (time) the pump current must reach the small leak threshold 1:

Small leak threshold 1 = idle current pump + K1 x (reference current - idle current). Factor K1 is between 0.16 and 0.28 depending on the characteristic current value of the pump (reference current - idle current), this value is various in every pump.

If the small leak threshold 1 is not reached in the small leak time, the small leak threshold 2 must be reached in an additional time small leak threshold 2 = reference current pump - K2 x (reference current - idle current). Factor K2 is between 0.60 and 0.80 depending on the characteristic current value of the pump (reference current-idle current).

If the small leak threshold 2 is also not reached, a leak > 0.04 inches is detected.

In the diagram below is the typical current of a tight system, a 0.02 inch leak, and a leak > 0.04 inches.

Fig 3: Typical Current Of A Tight System
Courtesy of BMW OF NORTH AMERICA, INC.
After the test the remaining pressure in the evaporative system is bled off through the charcoal canister by switching off the pump and solenoid.

Fig 4: Evaporative System Pressure Flow Diagram
Courtesy of BMW OF NORTH AMERICA, INC.