Evaporative Emissions System: Description and Operation

Fig. 20 Evaporative emission system. Non-turbocharged models (typical):

Fig. 21 Evaporative emission system. Turbocharged models (typical):

The Evaporative Emission System, Figs. 20 and 21, is used to prevent the escape of raw fuel vapors from the fuel system, into the atmosphere.
The system consists basically of carbon canister(s), a bowl vent valve, a purge control valve, a carbon element, a thermo valve, a vapor separator, a pressure relief fuel filler cap, an overfill limiter and a fuel check valve.
When the engine is off, fuel vapors from the fuel tank, carburetor float bowl (if equipped) and intake manifold are stored in the primary canister. When the engine is started, the vapors stored in the primary canister are routed to the secondary canister (if equipped), then to the purge control valve and on to the intake manifold, where they are burned. The purge control valve is closed during idle and opens when vacuum working on the valve diaphragm exceeds the pre-set value of the valve. The thermo valve closes the purge valve when coolant temperature is below the pre-set value of the valve, resulting in decreased levels of carbon monoxide and hydrocarbons during engine warm-up periods. When coolant temperature exceeds the calibration value of the thermo valve, the valve allows the purge control valve to open. The fuel vapors stored in the charcoal canister are routed directly into the carburetor or throttle body, while the vapors in the float bowl flow into the carburetor (if equipped) through the bowl vent valve. When intake manifold vacuum acting on the bowl vent valve diaphragm reaches a predetermined value, the bowl vent valve opens and allows fuel vapors to be drawn into the carburetor (if equipped) where they are routed through the intake manifold into the combustion chamber. The bowl vent valve is held in the open position, when the engine is operating, by the solenoid valve. When the engine is turned off, the bowl vent valve closes, allowing fuel vapors to be routed into the charcoal canister, where they are stored until the engine is restarted.
The pressure relief filler cap is equipped with a relief valve to prevent the escape of fuel vapor into the atmosphere. If the pressure in the tank drops below the specified negative pressure, the valve will open and adjust the pressure accordingly. The relief valve opening pressure is .640 ± .185 psi, while the open valve flow is 1.1 qts. at 1.3 inches Hg.
The vapor separator is used on some models to accommodate an increased volume of gasoline, when high ambient temperatures cause the gasoline to expand. It also helps prevent fuel from entering the vapor line during harsh cornering.
The overfill limiter is comprised of a pressure and vacuum valve. The pressure valve portion is designed to open when fuel tank internal pressure is above the specified norm, while the vacuum valve opens when excessive vacuum has been produced in the fuel tank.
The fuel check valve is used to prevent fuel leaks, should the vehicle accidentally roll over. The valve contains two check balls. During normal operation, the gasoline vapor passage in the check valve is open. However, should the vehicle roll over, one check ball moves to block the passage and prevent fuel from leaking out.