Evaporative Emissions System: Description and Operation

Vapor Canister With Non-encapsulated Solenoid:

SYSTEM DESCRIPTION

The Evaporative Emission Control System (EECS) 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. This retains the fuel 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.

Evaporative Emission Control System Schematic:

VAPOR CANISTER AND PURGE OPERATION

The venting of vapors to the canister is controlled by a diaphragm operated control valve, combined with a solenoid to control purging. When the engine is running, the ported vacuum supplied at the top of the purge valve allows the valve diaphragm to open. The lower port on the purge valve is connected to the solenoid valve.

The ECM operates a normally opened Pulse Width Modulated solenoid valve which controls vacuum to the purge valve in the charcoal canister. Under cold or idle conditions, the solenoid is turned "ON" by the ECM which blocks vacuum to the canister purge valve.
The ECM turns off the solenoid valve and allows it to purge when all of the following have been established:

1. The engine is warm.
2. After the engine has been running a specified time.
3. Above a specified road speed.
4. Above a specified throttle opening.

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

Tank Pressure Control Valve:

FUEL TANK PRESSURE CONTROL VALVE

The Fuel Tank Pressure Control Valve is located near the canister, and is connected to the fuel tank vapor line. With the engine running, manifold vacuum is supplied to the control vacuum tube and the valve is open. Vapors are routed to the tank when the engine is not running.