Leak Detection Pump System: Notes

The Evaporative (EVAP) emission system is designed to prevent the escape of fuel vapors from the fuel system. See Fig 1 . Leaks in the system, even small ones, can allow fuel vapors to escape into the atmosphere. Government regulations require on-board testing to make sure that the EVAP system is functioning properly. The leak detection system tests for EVAP system leaks and blockage. It also performs self-diagnostics. During self-diagnostics, the Powertrain Control Module (PCM) first checks the Leak Detection Pump (LDP) for electrical and mechanical faults. The LDP is located below driver's side of steering gear, above engine cradle. See Figure or Fig 2 . If the first checks pass, the PCM then uses the LDP to seal the vent valve and pump air into the system to pressurize it. If a leak is present, the PCM will continue pumping the LDP to replace the air that leaks out. The PCM determines the size of the leak based on how fast/long it must pump the LDP as it tries to maintain pressure in the system. EVAP leak detection system components are described as follows.

1. Service Port
   
   Used with special tools like the Miller Evaporative Emissions Leak Detector (EELD) to test for leaks in the system.
2. EVAP Purge Solenoid
   
   The PCM uses the EVAP purge solenoid to control purging of excess fuel vapors stored in the EVAP canister. It remains closed during leak testing to prevent loss of pressure.
3. EVAP Canister
   
   The EVAP canister stores fuel vapors from the fuel tank for purging.
4. EVAP Purge Orifice
   
   Limits purge volume.
5. EVAP System Air Filter
   
   Provides air to the LDP for pressurizing the system. It filters out dirt while allowing a vent to atmosphere for the EVAP system.

The main purpose of the LDP is to pressurize the fuel system for leak checking. It closes the EVAP system vent to atmospheric pressure so the system can be pressurized for leak testing. The diaphragm is powered by engine vacuum. It pumps air into the EVAP system to develop a pressure of about 7.5 in. H2O (1/4 psi). A reed switch in the LDP allows the PCM to monitor the position of the LDP diaphragm. The PCM uses the reed switch input to monitor how fast the LDP is pumping air into the EVAP system. This allows detection of leaks and blockage. The LDP assembly consists of several parts. See Fig 3 . The solenoid is controlled by the PCM and it connects the upper pump cavity to either engine vacuum or atmospheric pressure. A vent valve closes the EVAP system to atmosphere, sealing the system during leak testing. The pump section of the LDP consists of a diaphragm that moves up and down to bring air in through the air filter and inlet check valve, and pump it out through an outlet check valve into the EVAP system. The diaphragm is pulled up by engine vacuum, and pushed down by spring pressure as the LDP solenoid turns on and off. The LDP also has a magnetic reed switch to signal diaphragm position to the PCM. When the diaphragm is down, the switch is closed, which sends a 12-volt (system voltage) signal to the PCM. When the diaphragm is up, the switch is open, and there is no voltage sent to the PCM. This allows the PCM to monitor LDP pumping action as it turns the LDP solenoid on and off.