Evaporative Emissions System: Description and Operation

PURPOSE
The evaporative control system prevents the escape of fuel vapors from the fuel tank and fuel injection into the atmosphere.

BASIC OPERATION
Vapors generated in the fuel tank enter the charcoal canister where the charcoal absorbs and stores the fuel vapor.

When the ECM senses the correct conditions for operation, it supplies the ground circuit to the solenoid. This opens the solenoid, allowing manifold vacuum to be applied to the carbon canister, drawing the vapors out of the canister, and fresh air into the bottom of the canister.

When grounded, the purge solenoid redirects manifold vacuum to the lower vacuum chamber of the dual control purge valve. Vacuum pulls the valve open.

Vacuum from the air cleaner is used to purge turbo systems. It allows the canister to be purged under a vacuum or a boost condition.

When the purge solenoid is open (ungrounded), the ports of the purge solenoid are connected, causing equal boost pressures to be applied to both nipples of the purge valve. This causes the spring to push the diaphragm and the valve closed, shutting off the canister purge. The same thing happens at wide open throttle or heavy loads.


OPERATING CONDITIONS
The canister is purged (solenoid energized) under the following conditions:

CONDITION 1
Engine temperature above 158°F (70°C).
For the first three minutes after starting engine.
Engine rpm over 3000

CONDITION 2
Engine temperature above 158°F (70°C).
After three minutes of engine operation, the canister will be purged intermittently.
Engine rpm over 3000

CONDITION 3
Engine temperature above 158°F (70°C).
Altitude above 7,200 ft. (2,200 m).
Engine rpm over 3000

CONDITION 4
Engine temperature above 158°F (70°C).
Intake air temperature is above 122°F (50°C).
Engine rpm over 3000