What does the leak detection pump do?

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EVAP Leak Detection System Components

    Service Port: used with special tools such as the Miller Evaporative Emission Leak Detector (EELD) to test for leaks in the system.
    EVAP Purge Solenoid: The PCM uses the EVAP purge solenoid to control the purging of excess fuel vapors stored in the EVAP canister. It remains closed during leak tests to prevent pressure loss.
    EVAP Canister: The EVAP canister stores fuel vapors from the fuel tank for purging. EVAP Purge Orifice: limits the purge volume.
    EVAP System Air Filter: supplies air to the LDP to pressurize the system. It filters out dirt while providing a vent to the atmosphere for the EVAP system.

Leak Detection Pump (LDP) Components

The main purpose of the LDP is to pressurize the fuel system for leak checking. It seals the EVAP system vent to atmospheric pressure so the system can be pressurized for leak tests. The diaphragm is powered by engine vacuum. It pumps air into the EVAP system to develop a pressure of approximately 7.5 ‘H20 (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 the rate at which the LDP pumps air into the EVAP system. This allows for the detection of leaks and blockages.

The LDP assembly consists of several parts. The solenoid is controlled by the PCM and connects the upper pump cavity to engine vacuum or atmospheric pressure. A purge valve closes the EVAP system to the atmosphere, sealing the system during leak tests. The pump section of the LDP consists of a diaphragm that moves up and down to draw air through the air filter and inlet check valve and pump it through an outlet check valve into the EVAP system.

  The diaphragm is pulled upward by engine vacuum and pushed downward by spring pressure, when the LDP solenoid is turned on and off. The LDP also has a magnetic reed switch to signal the diaphragm position to the PCM. When the diaphragm is down, the switch is closed, sending a 12 V (system voltage) signal to the PCM. When the diaphragm is up, the switch is open and no voltage is sent to the PCM. This allows the PCM to monitor the LDP pumping action as it turns the LDP solenoid on and off.

When the LDP is at rest (no power/vacuum), the diaphragm may lower if the internal pressure (EVAP system) is not greater than the return spring. The LDP solenoid blocks the engine vacuum port and opens the atmospheric pressure port connected via the EVAP system air filter. The purge valve is held open by the diaphragm. This allows the canister to see atmospheric pressure.
Diaphragm Upward Movement

When the PCM energizes the LDP solenoid, the solenoid blocks the atmospheric port leading through the EVAP air filter and simultaneously opens the engine vacuum port to the pump cavity above the diaphragm. The diaphragm moves upward when the vacuum above the diaphragm exceeds the spring force. This upward movement closes the purge valve. It also causes a low pressure below the diaphragm, unseating the inlet check valve and allowing air to enter through the EVAP air filter. When the diaphragm completes its upward movement, the LDP reed switch changes from closed to open.

Based on the reed switch input, the PCM de-energizes the LDP solenoid, causing it to block the vacuum port and open the atmospheric port. This connects the upper pump cavity to the atmosphere through the EVAP air filter. The spring is now able to push the diaphragm downward. The downward movement of the diaphragm closes the inlet check valve and opens the outlet check valve, pumping air into the evaporative system. The LDP reed switch changes from open to closed, allowing the PCM to monitor the LDP pumping activity (diaphragm up/down). During the pumping mode, the diaphragm will not drop far enough to open the purge valve.

The pumping cycle is repeated as the solenoid is turned on and off. As the evaporative system begins to pressurize, the pressure at the bottom of the diaphragm will begin to oppose the spring pressure, slowing the pumping action. The PCM observes the time from when the solenoid is turned off until the diaphragm falls far enough for the reed switch to change from open to closed. If the reed switch changes too quickly, a leak may be indicated. The longer it takes for the reed switch to change state, the tighter the evaporative system is. If the system pressurizes too quickly, a restriction somewhere in the EVAP system may be indicated.

During certain parts of this test, the PCM uses the reed switch to monitor diaphragm movement. The solenoid is only energized by the PCM after the reed switch changes from open to closed, indicating the diaphragm has moved down. At other times during the test, the PCM will turn the LDP solenoid on and off rapidly to quickly pressurize the system. During a fast cycle, the diaphragm will not move enough to change the reed switch state. In the fast cycle state, the PCM will use a fixed time interval to cycle the solenoid.

The Duty Cycle EVAP Purge Solenoid (DCP) regulates the flow of vapor from the EVAP canister to the intake manifold. The Powertrain Control Module (PCM) operates the solenoid.

During the cold start warm-up period and hot start time delay, the PCM does not energize the solenoid. When de-energized, no vapor is purged. The PCM de-energizes the solenoid during open loop operation.

The engine enters closed loop operation after reaching a specified temperature and the time delay ends. During closed loop operation, the PCM cycles (energizes and de-energizes) the solenoid 5 or 10 times per second, depending on operating conditions. The PCM varies the vapor flow rate by changing the solenoid pulse width. Pulse width is the length of time the solenoid is activated. The PCM adjusts the solenoid pulse width based on engine operating conditions.