Leak Diagnosis Pump (LDP System)

Starting with the 98 model year the LDP method of evaporative system leak detection was introduced on E38 and E39 vehicles.

Components: 

Fig 1: Identifying Leak Diagnosis Pump (LDP System)

Courtesy of BMW OF NORTH AMERICA, INC.

Functional Overview: 

• The function of the LDP is to pressurize the fuel tank and the evaporative emission system to detect leaks. The pump also serves as the fresh air inlet path during normal purge operation when leak diagnosis is not occurring.
• The pump contains a spring loaded diaphragm which is moved up and down by solenoid controlled engine vacuum to generate the air pressure
  Fig 2: Identifying Working Of LDP (1 Of 3)

  Courtesy of BMW OF NORTH AMERICA, INC.
• During a leak test, the normally open vent valve is sprung closed to retain the built up pressure.
• The purge valve(s) are also sprung closed to seal the system.
• The reciprocation of the diaphragm pulls in filtered ambient air and pumps it into the fuel system via the purge canister as the vacuum supply is repetitively opened and closed electrically by the ECM.
  Fig 3: Identifying Working Of LDP (2 Of 3)

  Courtesy of BMW OF NORTH AMERICA, INC.
• The ECM monitors the diaphragm movement through a reed contact feedback signal and compares it to its activation output frequency of the vacuum solenoid in the LDP.
• As the pump continues to operate the diaphragm begins to slow down against the built up pressure in the system. The time delay between the vacuum solenoid activation and the reed contact feedback is the basis for leak detection.

Fig 4: Identifying Working Of LDP (3 Of 3)

Courtesy of BMW OF NORTH AMERICA, INC.

• If the reed contact feedback signal slows down considerably this indicates the pressure is being held by the system and no leaks are present.
• If the reed contact feedback signal is slowed down but not to the satisfaction of a sealed system the ECM will determine a small leak is present.
• If there is no delay in the feedback signal the ECM determines a large leak is present (ie: missing fuel filler cap).