Evaporative Fuel System Pressure Leak Diagnosis

M5.2.1 Equipped Vehicles (and 1998 528i vehicles with MS 41.1 as of 9/97 prod.)












To meet the next level of emission stage compliancy, a Leak Diagnosis Pump (LDP) is installed for fuel system evaporative leak testing.

The vacuum system introduced on the TLEV 328i was the first system capable of detecting evaporative system leaks. It is capable of detect a leak as small as 1.0 mm (0.040").

The LDP equipped system is capable of detecting a leak as small as 0.5 mm.(0.020").

The LDP is located in the left rear (driver's side) fender well. The LDP is a unitized component that contains the following:
- Vacuum chamber,
- Pneumatic pressure chamber;
- DME activated vacuum solenoid,
- Reed switch providing a switched voltage feedback signal to the DME.

The LDP is only replaceable as a complete component.

The vacuum supply line is in the wiring harness from the engine compartment and runs down the driver's side of the vehicle.

In it's inactive state, the LDP diaphragm is at the bottom end of it's down stroke. The diaphragm pushes a rod downward against spring pressure to open the canister vent valve.

This open valve serves as the filtered air inlet path for the evaporative fuel system to compensate for engine vacuum during purge operation and fuel consumption as the vehicle is driven.












- The function of the LDP is to pressurize the fuel tank and the evaporative emission system (approx. 25mb.) for the purpose of detecting leaks.
- During diagnostic testing of the evaporative emission system, the vent valve is closed and will block atmospheric venting. The purge valves are also sprung closed to seal the system.