GM Astro/Safari 1985-1996 Repair Guide

Early Fuel Evaporation System



See Figures 1, 2, 3 and 4

The Early Fuel Evaporation (EFE) system, used on carbureted models, consists of an EFE valve at the flange of the exhaust manifold, an actuator, and a thermal vacuum switch. The TVS is located in the coolant outlet housing and directly controls vacuum.

In both systems, manifold vacuum is applied to the actuator, which in turn, closes the EFE valve. This routes hot exhaust gases to the base of the carburetor. When coolant temperatures reach a set limit, vacuum is denied to the actuator allowing an internal spring to return the actuator to its normal position, opening the EFE valve.

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Fig. Fig. 1: Valve, actuator and TVS-4.3L carbureted engine

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Fig. Fig. 2: Thermal vacuum switch port identification

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Fig. Fig. 3: Thermal vacuum switch location-49 state models

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Fig. Fig. 4: Thermal vacuum switch location-California models


  1. Locate the EFE valve on the exhaust manifold and note the position of the actuator arm. On some vehicles, the valve and arm are covered by a two-piece cover which must be removed for access. Make sure the engine is overnight cold.
  3. Watch the actuator arm when the engine is started. The valve should close when the engine is started cold; the actuator link will be pulled into the diaphragm housing.
  5. If the valve does not close, stop the engine. Remove the hose from the EFE valve and apply 10 in. Hg (33.8 kPa) of vacuum by hand pump. The valve should close and stay closed for at least 20 seconds (you will hear it close). If the valve opens in less than 20 seconds, replace it. The valve could also be seized if it does not close; lubricate it with spray type manifold heat valve lube. If the valve does not close when vacuum is applied and when it is lubricated, replace the valve.
  7. If the valve closes, the problem is not with the valve. Check for loose, cracked, pinched or plugged hoses, and replace as necessary. Test the EFE solenoid (located on the valve cover bracket); if it is working, the solenoid plunger will emit a noise when the current is applied.
  9. Warm up the engine to operating temperature.
  11. Watch the EFE valve to see if it has opened. It should now be open. If the valve is still closed, replace the solenoid if faulty, and/or check the engine thermostat; the engine coolant may not be reaching normal operating temperature.


See Figure 5

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Fig. Fig. 5: EFE valve and actuator assembly mounting

If the vehicle is equipped with an oxygen sensor, it is located near the EFE valve. Use care when removing the EFE valve as not to damage the oxygen sensor.

  1. Disconnect the negative (-) battery cable and vacuum hose at the EFE valve.
  3. Remove the exhaust pipe-to-manifold nuts, and the washers and tension springs if used.
  5. Lower the exhaust cross-over pipe. On some models, complete removal of the pipe is not necessary.
  7. Remove the EFE valve.

To install:

Always install new seals and gaskets. Torque the exhaust nuts to 15 ft. lbs. (20 Nm). Connect the negative battery cable and vacuum hose to the valve.