The evaporative emission control system prevents gasoline vapor emissions from the fuel system from entering the atmosphere.
Evaporating fuel from the gas tank or carburetor, passes through vent hoses and tubes to a charcoal canister where they are temporarily stored until they can be drawn into the intake manifold and burned when the engine is running.
The charcoal canister is a sealed, maintenance-free unit which stores fuel vapors from the fuel tank and carburetor bowl. Although all carburetor bowls are vented internally, some models do not required venting to the canister. In cases where the carburetor is not vented to the canister, the bowl vent port on the canister will be capped. If the canister becomes damaged, replacement with a new unit is required. The hoses connecting the canister are of fuel resistant construction. Use only fuel resistant hoses if replacement is necessary.
Some models are equipped with a damping canister that is connected in series with the charcoal canister. The damping canister cushions the effect of a sudden release of fuel rich vapors when the purge valve is signaled to open. The rich vapors are held momentarily and then gradually fed into the intake manifold to be burned.
CANISTER PURGE SOLENOID
See Figure 4
All engines, except 1994-95 vehicles equipped with 3.0L, 3.3L and 3.8L engines, are equipped with a canister purge solenoid which is connected in series with the charcoal canister. The canister purge solenoid is electrically operated by the engine computer (SMEC, SBEC, SBEC II or PCM), which grounds the solenoid when the engine temperature is below 151°F (66°C). This prevents vacuum from reaching the charcoal canister. When the engine reaches operating temperature the SMEC de-energizes the solenoid and allows purge vapors from the canister to pass through the throttle body.
DUTY CYCLE CANISTER PURGE SOLENOID
See Figure 5
All 1994-95 vehicles equipped with 3.0L, 3.3L and 3.8L engines are equipped with a duty cycle EVAP canister purge solenoid. The duty cycle EVAP canister purge solenoid regulates the rate of vapor flow from the EVAP canister to the throttle body and is controlled by the Powertrain Control Module (PCM).
ROLL OVER VALVE
All vehicles must pass a full 360° roll over test without allowing fuel leakage. To keep fuel from leaking when the vehicle is inverted, fuel and vapor flow controls are needed for all fuel tank connections. A roll over valve is mounted in the top of the fuel tank to prevent leakage if the vehicle is involved in a roll over.
GAS TANK FILLER CAP
See Figure 6
The fuel tank is covered and sealed with a specially engineered pressure/vacuum relief gas cap. The built-in relief valve is a safety feature, and allows pressure to be relieved without separating the cap from the filler tube while eliminating excessive tank pressure. If a replacement cap is required, a similar cap must be installed in order for the system to remain effective.
Always remove the gas tank cap to release pressure whenever the fuel system requires servicing.
BOWL VENT VALVE
The bowl vent valve (carburetor equipped models) is connected to the carburetor fuel bowl, the charcoal canister, and the air pump discharge. When the engine is not running and no air pump pressure is applied, a direct connection between the carburetor and canister exists. When the engine is running, air pump pressure closes the connection between the canister and the fuel bowl. When the engine is shut off, air pressure in the valve bleeds down and the fuel bowl is allowed to vent vapors into the canister.