The evaporative emission system is designed to prevent the escape of fuel vapors from the fuel
system. Leaks in the system, even small ones, can allow fuel vapors to escape into the
atmosphere. Government regulations require onboard Testing to make sure that the evaporative
(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.
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.
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 0.25 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. 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 12v (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.

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