EVAP System Operation

The ECM monitors the control circuits to the vent solenoid, purge solenoid, and fuel tank pressure sensor. In addition, the ECM monitors the ability of the system to maintain vacuum. Under normal operation the vent valve remains open. This prevents excessive pressure buildup in the fuel tank and EVAP canister. When specific conditions are met, the ECM supplies a ground to energize/open the purge valve, which is normally closed. The vacuum in the intake manifold causes the fuel vapors in the EVAP canister to be purged. This vacuum is also drawing in fresh air through the vent valve as well as the standard intake. The ground from the ECM is Pulse Width Modulated (PWM), pulsed on and off several times per second, to control the flow of the vapors. The ECM determines when and for how long the purge valve will be open based on engine load, throttle position, coolant temperature, and ambient temperature.