Evaporative Emissions System: Description and Operation

Evaporative Emissions Control Schematic:

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 is consumed in the normal combustion process.

Inverted Function Fuel Vapor Canister:

FUEL VAPOR CANISTER

Gasoline vapors, from the fuel tank, flow into the canister tube labeled "TANK". Any liquid fuel present flows into a reservoir at 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 vapor canister through the top air tube. The air mixes with the vapor and is drawn into the intake manifold to be burned.


EVAPORATIVE CONTROL

The ECM operates a solenoid valve which controls vacuum to the purge passage of the charcoal canister. Under cold or idle conditions, the solenoid is turned "OFF" by the ECM which blocks vacuum to the canister purge passage. An increase in fuel vapor purging is controlled by the Oxygen Sensor signal. If a rich condition is sensed by the Oxygen Sensor, the purge solenoid is turned "OFF" by the ECM.
The ECM turns "ON" the solenoid valve and allows purging when all of the following have been established:

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

Tank Pressure Control Valve:

FUEL TANK PRESSURE CONTROL VALVE

The fuel Tank Pressure Control (TPC) valve is a combination roll-over, integral pressure, and vacuum relief valve. When vapor pressure in the tank exceeds 1.5" Hg (5 kPa) the valve opens and allows vapors to vent to the canister, and then the vapors are purged. When the tank pressure drops below the opening point of the valve, it will close. This allows fuel vapors to be trapped in the fuel tank. The TPC valve contains an umbrella valve that opens when the fuel tank reaches approximately 0.5" Hg (1.6 kPa) of vacuum. The TPC 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.