Evaporative Emissions System: Testing and Inspection

Chart C-3 Evaportive Emission Control:

Chart C-3 Wiring Diagram:

CIRCUIT DESCRIPTION:
The canister purge is controlled by a solenoid that allows manifold vacuum to purge the fuel vapor canister when de-energized. The ECM supplies a ground to energize the solenoid (purge "OFF"). The purge solenoid controlled by the ECM is pulse width modulated (turned "ON" and "OFF" several times a second). The duty cycle (pulse width) is determined by the amount of air flow, and the engine vacuum as determined by the MAP sensor input. The duty cycle is calculated by the ECM and the output commanded when the following conditions have been met:

^ The engine run time after start is more than 3 minutes.
^ The coolant temperature is above 80°C (176°F).
^ Vehicle speed above 15 mph.
^ Throttle at idle (about 3%).

Also, if the diagnostic "test" terminal is grounded with the engine stopped, the purge solenoid is de-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. The solenoid is normally energized in this step; so it should be closed.

2. Checks for a complete circuit. Normally there is ignition voltage on CKT 250 and the ECM provides a ground on CKT 428.

3. Completes functional check by grounding the "test" terminal. This should normally de-energize the solenoid opening the valve which should allow the vacuum to drop (purge "ON").

DIAGNOSTIC AIDS:
Normal operation of the canister purge solenoids described as follows:

^ With ignition "ON" engine "OFF" diagnostic "test" terminal ungrounded, the canister purge solenoid will be energized.
^ With ignition "ON" engine "OFF" diagnostic "test" terminal grounded, the canister purge solenoid will be de-energized.

An inoperative canister purge system can cause rich exhaust condition Code 45. If the solenoid valve should become stuck open or the carbon canister become saturated, a continuous rich fuel condition may result.