Evaporative Emissions System: Testing and Inspection

Chart C-3:

EVAP Solenoid Circuit:

CIRCUIT DESCRIPTION:
The canister purge is controlled by a solenoid that allows manifold vacuum and/or ported vacuum to purge the fuel vapor canister when energized. The ECM supplies a ground to energize the valve (purge "ON"). The purge solenoid valve controlled by the ECM is pulse width modulated (turned "ON" and "OFF" several times a second). The duty cycle (pulse width) is determined by "CLOSED LOOP" feed back from the oxygen sensor. The duty cycle is calculated by the ECM and the output commanded when the following conditions have been met:

^ The engine run time is more than 65 seconds.
^ The coolant temperature is above 56°C (133°F).

Also, if the diagnostic test terminal is grounded with the engine stopped, the purge solenoid is energized (purge "ON").

TEST DESCRIPTION: The numbers below refer to circled numbers on the diagnostic chart.

1. This checks to see if the solenoid is opened or closed. It should be closed (de-energized).

2. This determines if the solenoid was open due to an electrical circuit problem or a defective solenoid.

3. This completes the functional check by grounding the test terminal. This should normally energize the solenoid and allow the vacuum to drop (purge "ON").

DIAGNOSTIC AIDS:
Make a visual check of vacuum hose(s). Check the throttle body for a possible cracked, broken, or plugged vacuum block. Check engine for possible mechanical problem.