Operation

The Powertrain Control Module (PCM) monitors the ability of the system to maintain vacuum. If the vacuum remains for a specified period of time, then there are no evaporative leaks and a Pass is reported to the diagnostic executive. If there is a leak, the system either will not achieve a specific vacuum or a vacuum cannot be maintained. A fault can only be detected after a cold start with a trip of sufficient length and driving conditions to run the needed tests.

The canister purge solenoid allows intake manifold pressure (vacuum) to purge the stored vapors in the EVAP canister. The PCM supplies a ground to energize (open) the normally closed purged solenoid which allows vacuum to purge the canister. The ground is Pulse Width Modulated (PWM), pulsed on and off several times per second, to control the flow of purge vapors from the canister. The PCM calculates a PWM duty cycle for canister purge flow control using the following operating parameters: engine load, throttle position, coolant temperature and ambient temperature.