BMW 5251 Service Manual page 448

Fuel Injection
Evaporative fuel system pressure leak
diagnosis, overview (M5.2.1 only)
1998 540i models are equipped with an evaporative fuel sys-
tem pressure leak diagnosis system. The main component of
this system is the leak diagnosis pump (LDP). The function of
the LDP is to pressurize the fuel tank and the evaporative
emission system for the purpose of detecting leaks. This sys-
tem is capable of detecting a leak as small as 0.5 mm.
The LDP, located in the left rear wheel housing, is a unitized
component that contains the following:
Vacuum chamber.
Pneumatic pump chamber.
DME activated vacuum solenoid
Reed switch (provides switched voltage feedback signal to
ECM).
The vacuum supply line is in the wiring harness from the en-
gine compartment and runs down the driver's side of the vehi-
cle.
The canister vent valve is integrated into the LDP, which is
electrically controlled by the ECM. The canister vent valve is
opened to provide fresh air entry into the fuel system only dur-
ing purge operation.
-
Purge operation characteristics:
O f f
idle through full throttle: purge valve opened by pulse
width modulated (PWM) control of the ECM. PWM duty cy-
cle varies by engine operating conditions (e.g. warm idle,
purge valve opened slightly; cold idleldecel with engine
temp < 67" C, purge valve always closed).