OperationFuel Tank Venting
Trapped fuel vapors inside the fuel tank are vented through an orifice to the vapor valve assembly on top of the tank. These vapors leave the valve assembly through a single vapor line and continue to the canister, for storage, until they are purged to the engine for burning.Carburetor Venting
The vapors from the fuel bowl are vented to the carbon canister when the engine is stopped. When the engine is started and a specified engine temperature is reached, the vapors will be drawn into the engine for burning. These vapors are controlled by the canister purge solenoid, the canister purge valve, the carburetor fuel bowl solenoid vent valve and the carburetor fuel bowl thermal vent valve (if used).Canister Purging
See Figure 1
Purging the carbon canister removes the fuel vapor stored in the carbon. With a computer controlled EVAP system, the flow of vapors from the canister to the engine is controlled by a purge solenoid (CANP). Others use a vacuum controlled purge valve. Purging occurs when the engine is at operating temperature and off idle.
This component is a heater that warms the air/fuel mixture below the carburetor for better fuel evaporation when the engine is cold. The fuel evaporation heater consists of a spacer, upper and lower gaskets and a 12 volt grid-type heater attached to the bottom side of the primary bore of the spacer. The offset design of the heater mounting bracket positions the heater in the intake manifold inlet opening.Fuel Evaporative Heater Switch
The evaporative heater switch is mounted at the rear of the engine, on the bottom of the intake manifold. It controls the relay and the heater element in the early fuel evaporative emission system, based on engine temperature. The normally closed switch will activate the relay and the heater at low engine temperature and will open at the specified calibration of the temperature switch. This will open the control relay, which in turn will shut off the early fuel evaporation heater after the engine has warmed up.Fuel/Vacuum Separator
The fuel/vacuum separator is used in carbureted systems in order to prevent fuel travel to a vacuum operated device. This component requires positive orientation to insure that any fuel collected will drain back to the carburetor. If the separator becomes clogged or cracked, it must be replaced.
TestingThermostatic Bowl Vent Valve
See Figure 2
- Check the vacuum vent valve, at engine temperatures of 120¡F (49¡C) or more. Air should flow between the carburetor port and canister port when no vacuum is applied to the vacuum signal nipple.
- It should not allow the flow of air with a vacuum applied at the vacuum signal nipple.
- At a temperature of 90¡F (32¡C) or less, the valve should have an air flow or be very restrictive to air flow.
See Figure 3
The vacuum bowl vent valve should have an air flow between the carburetor port and the canister port when no vacuum is applied to the vacuum signal nipple, and should not have an air flow with vacuum applied at the vacuum signal nipple.
See Figure 4
- Apply vacuum to port A (only), there should be no air flow. If air flow occurs, replace the valve.
- Apply vacuum to port B (only), there should no air flow. The valve should be closed. If air flow occurs, replace the valve.
- Apply and maintain 16 in. Hg (110 kPa) of vacuum to port A, and apply vacuum to port B. Air should pass.
See Figure 5
Apply 9-14 volts DC to the fuel bowl vent solenoid valve. The valve should close, not allowing air to pass. If the valve does not close or leaks when voltage and 1 in. Hg (68 kPa) of vacuum is applied to the carburetor port, replace the valve.
- When the engine coolant temperature is below 128¡F (53¡C), the switch should be closed.
- When the heater relay is energized, the relay contacts close allowing current to flow through the relay and to the heater.
- The heater operates for approximately the first three minutes of cold engine operation which aids in a leaner choke calibration for improved emissions without cold drive-away problems.
- At ambient temperatures of less than 40¡F (4¡C), the leaner choke calibrations reduce loading and spark plug fouling.
- The heater grid is functioning if radiant heat can be detected when the heater grid is energized.
Do not probe the heater grid while the grid is in the heat mode, as it is possible to cause a direct short in the circuit. The heater is designed to operate at a constant temperature of approximately 320-383ºF (160-195ºC), and could result in burns if touched.
REMOVAL & INSTALLATION
Removal and installation of the evaporative emission control system components consists of locating the component, labeling and disconnecting hoses, loosening retaining screws and removing the part which is to be replaced from its mounting point.
When replacing any EVAP system hose, always use hoses that are fuel-resistant or marked EVAP. Use of hoses which are not fuel-resistant will lead to premature hose failure.