Evaporative Emissions System: Description and Operation

Evaporative Emissions 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:

VAPOR CANISTER
The canister has a diaphragm operated purge valve and a solenoid to control purge. When the engine is running, port manifold vacuum is supplied to the top of the purge valve (Control Vacuum Signal) which lifts the valve diaphragm and opens the valve when the throttle is above a specified opening. The lower tube on the purge valve (PCV tube) is connected to the solenoid valve.

EVAPORATIVE CONTROL
This system has a remote mounted purge control solenoid. The ECM operates this solenoid to control vacuum to the canister purge valve. 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.
The Control valve only opens when enough vacuum is supplied. This prevents purging under conditions of low ported vacuum.

Fuel Tank Pressure Control Valve:

FUEL TANK PRESSURE CONTROL VALVE
This control valve is a, normally closed, spring-biased vacuum diaphragm valve. When the engine is running and the throttle is above a specified opening, vacuum is applied to the valve and it opens. Fuel vapors are now able to vent to the canister. When the engine is turned OFF or the throttle is closed below the specified point, no vacuum is applied to the valve, and the fuel tank is vented through the restriction vent. This restriction will hold vapor in the tank rather than allowing the vapor into the canister.