Evaporative Emissions System: Description and Operation

Evaporative emission control system:

SYSTEM DESCRIPTION

To prevent the loss of fuel vapor from the fuel system into the atmosphere a closed evaporative emission-control system, is used to recover and burn fuel system vapors.

Typical charcoal canister:

CARBON CANISTER

While the engine is not in operation fuel vapors generated inside the fuel system are absorbed and stored in the carbon canister. While the engine is running fuel vapors absorbed in the canister are drawn into the intake manifold through the purge-control valve, and an orifice.

Purge control valve.:

PURGE CONTROL VALVE

The purge control solenoid valve, is controlled by the ECU.

Purge control system:

When engine coolant temperature is low, or during idling, the purge control valve remains closed preventing fuel vapors from entering the surge tank and reducing HC emissions.

After engine warm-up, during ordinary driving the ECU opens the purge control valve to purge fuel vapors from the fuel system into the surge tank.

Fig. 48 Thermovalve:

THERMO VALVE (FEDERAL AND CANADA ONLY)

The thermo valve is used to prevent purge system operation until the engine has warmed up (coolant temperature greater than 45 degrees C or 113 degrees F). This prevents driveability problems and excessive hydrocarbon emissions during engine warm up

EGR TEMPERATURE SENSOR (CALIFORNIA ONLY)

The EGR temperature sensor (thru computer control) is used to prevent purge system operation until the engine has warmed up (coolant temperature greater than 45 degrees C or 113 degrees F). This prevents driveability problems and excessive hydrocarbon emissions during engine warm up.

Fuel Filler Cap Operation:

FUEL FILLER CAP

The fuel filler cap, is equipped with a relief valve to prevent the escape of fuel vapors to the atmosphere and to prevent the collapse of the fuel tank under negative pressure.

Overfill Limiter (Two Way Valve):

OVERFILL LIMITER

The overfill limiter, consists of a pressure valve and a vacuum valve. The pressure valve is designed to open when the fuel tank internal pressure has increased over normal pressure and the vacuum valve opens when a vacuum has been produced in the tank.

Fuel Check Valve:

FUEL CHECK VALVE

The fuel check valve, is used to prevent fuel leaks should the car suddenly roll over. This valve is connected in the fuel vapor line (between the canister and overfill limiter) and is mounted on the firewall.

The fuel check valve contains two balls. Under normal conditions, the gasoline vapor passage in the valve is opened, but if roll over occurs one of the balls closes the fuel passage preventing fuel leaks.