Evaporative Emissions System: Description and Operation

Evaporative Emission Control System 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 consumed in the normal combustion process.

Fig. 59 Inverted function vapor canister w/encapsulated purge solenoid:

FUEL VAPOR CANISTER
Gasoline vapors from the fuel tank flow into the tube labeled "TANK". Any liquid fuel goes 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.

Tank Pressure Control Valve:

EVAPORATIVE CONTROL
The ECM operates a pulse width modulated solenoid valve which controls vacuum to the purge valve in the charcoal canister. Under cold engine or idle conditions, the solenoid is turned off by the ECM, which closes the solenoid and blocks the vacuum to the canister purge valve. The feedback system, used by the ECM, can sense a rich condition from the O2 sensor. The purge is regulated until the ECM no longer receives a rich signal from the O2 sensor.

The canister is commanded by the ECM to purge when all of the following have been established:

1. Above a specified road speed
2. Engine is warm
3. The engine must have been running for a specified time
4. Above a specified throttle opening


FUEL TANK PRESSURE CONTROL VALVE
The fuel tank pressure control valve is a combination roll-over, integral pressure, and vacuum relief valve. When vapor pressure in the tank exceeds 1" Hg (5 kPa) the valve opens and allows vapors to vent to the canister, and then 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 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.