Evaporative Emissions System: Description and Operation

Evaporative Emission Control System:

The evaporative emission control system (EECS) consists of the fuel tank and filler cap, a check/relief valve, two roll-over/float valves, a vacuum switching valve and charcoal canister with integral purge control valve. The system collects fuel vapors that would otherwise be discharged into the atmosphere and temporarily stores them in a charcoal canister. When the engine is started, the vapors are directed to the intake to be burned in the engine. Canister purge is ECM controlled by means of a vacuum switching valve. This system of vapor recovery helps prevent raw hydrocarbon emissions and also promotes fuel economy by eliminating fuel loss through evaporation.

Canister And Purge Valve:

PURGE VALVE

The purge valve is an integral part of the canister. The purge valve diaphragm is activated by a throttle vacuum signal.
When the engine is not running, a diaphragm spring holds the valve closed. When the engine is running and vacuum exceeds the specified value, the valve opens and fuel vapor is drawn into the intake manifold. Purge rate is regulated by intake manifold vacuum and the valve orifice.

Vacuum Switching Valve:

VACUUM SWITCHING VALVE

The vacuum switching valve is connected in series on the purge signal line to cut off vapor purge under conditions of low vehicle and engine speed. The valve is actuated by the ECM when necessary to maintain adequate driveability.

Check And Relief Valve:

CHECK AND RELIEF VALVE

When the engine is not running:
Fuel vapor from the rollover fuel trap is fed to the check and relief valve. When vapor pressure reaches 40 mmHg (1.6 in.Hg), the check valve opens to allow vapors into the canister.

When the engine is running:
When the pressure differential between the relief side and the fuel tank becomes 20 mmHg (0.8 in.Hg), the relief valve opens to allow air from the air cleaner into the fuel tank. This restores atmospheric pressure to the fuel tank.

Rollover And Float Valve:

ROLLOVER AND FLOAT VALVE

There are two rollover and float valves that prevent fuel leaks in the event of vehicle rollover.