DESCRIPTION
The evaporative emission control system is designed to prevent fuel vapors (hydrocarbons) from being emitted from the fuel system into atmosphere. The system consists of a sealed fuel tank, pressure/vacuum relief fuel cap, fuel tank vapor valve, vapor tube and hoses, and a carbon canister.
CARBURETED ENGINES
Carbureted engines are equipped with a fuel bowl solenoid vent valve, purge control valve, purge control solenoid valve, thermactor idle vacuum valve (TIV), thermal vent valve, vacuum bowl vent valve, and vacuum/thermostatic bowl vent valve. Carbon canister is purged by drawing vapors into intake manifold.
FUEL INJECTED ENGINES
Fuel injected engines are equipped with an in-line purge solenoid (all 1.6L and 2.3L Federal, as well as 5.0L California models), or a purge valve (all 3.8L and 5.0L Federal). Carbon canister is purged by drawing vapors into air cleaner.
Note. Not all of these components are used on any one system. Component usage depends on calibration of complete vehicle.
FILL CONTROL/VENT SYSTEM
Fill limiting is accomplished through configuration of fill neck and/or internal vent lines within fill neck and tank. Vent system is designed to permit air space in 10-12 percent of tank when tank is filled to capacity. Air space provides for thermal expansion of fuel as well as being an aid to in-tank vapor vent system.
VAPOR VENT SYSTEM
System provides a vapor space above gasoline surface in fuel tank. This area is sufficient to permit adequate breathing space for tank vapor valve assembly.
All vapor valves use a small orifice that allows only vapor and not liquid fuel to pass into line running to canister. Vapor valve is mounted on fuel tank using a rubber grommet.
Fuel vapors trapped in sealed fuel tank are vented though vapor valve assembly on top of fuel tank. Vapors are routed through a single vapor line to carbon canister in engine compartment.
Vapors are stored in carbon canister until they are purged into engine while engine is operating.
On vehicles equipped with fuel/vapor return lines, vapor generated in fuel supply line is continuously vented back to fuel tank. Venting prevents engine surging from fuel enrichment and assists in hydrocarbon emission control.
CARBON CANISTER
Fuel vapors from fuel tank and carburetor bowl are stored in carbon-filled canister. There are 2 different canister sizes used: .97 qts. (.93L) and 1.5 qts. (1.4L).
CARBURETOR VENTING
Fuel vapors which might otherwise collect in carburetor bowl and pass directly into atmosphere are vented to carbon canister when engine is stopped. Flow of vapors is controlled by a fuel bowl vent valve or a fuel bowl thermal vent valve.
When engine is started, vapors will be drawn into engine for burning. The time at which vapors are drawn into engine will depend on operating mode of engine, when purging system is most efficient.
PURGE CONTROL VALVE
Purge control valve, located in line with carbon canister, controls flow of fuel vapors from carbon canister to intake manifold. Air should not flow through PCV purge line port, unless more than 16 in. Hg is applied to carburetor vacuum port.
Purge Control Valve & Carbon Canister. Scheme 36
PURGE CONTROL SOLENOID VALVE
Purge control solenoid valve controls vapor flow from canister to intake manifold. Being normally closed, it is opened by a signal from Electronic Control Assembly (ECA).
FUEL BOWL SOLENOID VENT VALVE
Fuel bowl solenoid vent valve, located in fuel bowl vent line, is normally open. It is used on Motorcraft 7200 carburetors. These carburetors are equipped with a built-in fuel bowl vent valve.
When ignition switch is turned to the "ON" position, vent valve closes fuel bowl vent line and returns to normal open position when ignition switch is turned to the "OFF" position.
Note. If carburetor displays a lean air/fuel mixture condition, inspect either purge solenoid valve or fuel bowl solenoid valve for proper closing during engine operation.
Note. On EEC equipped vehicles with 7200 carburetors, a rich air/fuel mixture will result from an open or leaking valve.
Cutaway View of Fuel Bowl Solenoid Vent Valve. Scheme 37
FUEL BOWL THERMAL VENT VALVE
Thermal vent valve, located in carburetor-to-canister vent line, is closed when engine compartment temperature is cold. This prevents fuel tank vapors, generated when fuel tank heats up before engine compartment does, from being vented through carburetor fuel bowl.
THERMACTOR IDLE VACUUM VALVE
Thermactor Idle Vacuum valve (TIV) is used with some evaporative emission systems to improve idle quality at hot start. To overcome this poor idle condition, TIV valve bleeds air into PCV tube. This leans idle fuel mixture until purge valve opens. Same vacuum that opens purge valve closes TIV valve.
Installation of Thermactor Idle Vacuum Vent Valve (Typical). Scheme 38
VACUUM BOWL VALVE & VACUUM/THERMOSTATIC BOWL VALVE
Vacuum bowl valve and vacuum/thermostatic bowl valve, similar in appearance, are located in carburetor-to-canister vent line, and are vacuum and vacuum/temperature operated "ON/OFF" valves.
Vacuum bowl valve remains open, until manifold vacuum (when engine is operating) causes it to close. Vacuum/thermostatic bowl vent remains closed with temperature below 90° F (32° C) and open with temperature above 120° F (48° C), unless closed by manifold vacuum.
VACUUM CONTROL VALVE 2, 3 & 4-PORT
Vacuum control valves, attached to the engine coolant system, consist of 2, 3 or 4-ports, most utilizing wax pellet principle. Vacuum control valve turns purge valve vacuum on as engine warms up and allows it to turn off when engine is turned off.
A 4-port switch is same as two 2-port switches in 1 body. A 3-port switch moves vacuum source from top or bottom ports. Other types of switches are bi-metallic and electrical; electrical may by open or closed at room temperature and will switch when engine reaches full operating temperature.
PRESSURE/VACUUM RELIEF FUEL CAP
This system consists of a sealed filler cap with an integral pressure/vacuum relief valve. Fuel system vacuum relief is provide after 1.0 in. Hg of vacuum, and pressure relief after 1.8 psi.
Under normal conditions, fill cap allows air to enter fuel tank as fuel is used while preventing vapors from escaping.
FUEL VAPOR RETURN SYSTEM
This system consists of a vapor return line from fuel pump to fuel return outlet of fuel sender, reducing amount of fuel vapor entering carburetor.
AIR CLEANER ASSEMBLY
On some models, an auxiliary fuel bowl vent tube runs to air cleaner. An air filter installed on air cleaner end of fuel bowl vent tube keeps dirt out of carburetor fuel bowl.
TESTING
Note. Testing information not available.
MAINTENANCE
Replace charcoal canister filter every 30,000 miles. All hoses and connections should be checked periodically for cracks, leaks or other damage and replaced as necessary.