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 W/Non-Encapsulated Solenoid:

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
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 Vac. tube on the purge valve of the canister is connected to a ported vacuum source. When the engine is above idle speed, enough vacuum is available to open the purge valve diaphragm. Vapors are purged through the solenoid to the combustion chamber.

Fuel Tank Pressure Control Valve:

FUEL TANK PRESSURE CONTROL VALVE
A Tank Pressure Control Valve is also used to control the flow of vapors to the canister. It is located near the canister and is connected to the fuel tank vapor line. When the engine is running, manifold vacuum is supplied to the control vacuum tube and the valve is opened allowing fuel vapors to vent to the canister. When the engine is OFF, the valve closes and vapors remain in the tank.