Evaporative Emissions System: Description and Operation

Evaporative Emission Control System Schematic:

The evaporative emission control system used on this vehicle is the charcoal canister storage method. This method transfers fuel vapor from the fuel tank to an activated carbon storage canister to retain the vapors when the vehicle is not operating. When the engine is running the fuel vapor is purged from the carbon element by intake air flow and consumed in the normal combustion process.

Vapor Canister With Encapsulated Purge Solenoid:

VAPOR CANISTER
Gasoline vapors from the fuel tank flow into the tube labeled "Tank". Any liquid fuel goes into a reservoir in the bottom of the canister to protect the carbon bed above. These vapors are absorbed into the carbon. The canister is purged when the engine is running above idle speed. Ambient air is allowed into the canister through the air tube in the top. The air mixes with the vapor and is drawn into the intake manifold.

EVAPORATIVE CONTROL
The ECM operates a normally closed solenoid which controls vacuum to the purge valve in the charcoal canister. Under cold engine or idle conditions, the solenoid is turned "ON" by the ECM, which closes the solenoid and blocks vacuum to the canister purge valve. The ECM turns "OFF" the solenoid valve and allows purge when:

Engine is warm.
After the engine has been running a specified time.
Above a specified road speed.
Above a specified throttle opening.

IN-TANK PRESSURE CONTROL VALVE
The in-tank pressure control valve, a combination roll-over, integral pressure and vacuum relief valve, is located with the fuel sending unit in the fuel tank. When vapor pressure in the tank exceeds 1" Hg (5kPa) the valve opens and allows vapors to vent to the canister and then to be purged. When the tank pressure drops below the opening point of the valve it will close, keeping vapors in the fuel tank. The valve provides vacuum relief to protect against vacuum build up in the fuel tank and roll over protection to prevent liquid fuel from entering the canister during normal driving maneuvers.