GM Corvette 1963-1982 Repair Guide

Early Fuel Evaporation (EFE) System


See Figure 1

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Fig. Fig. 1: Common EFE valve mounting and related components

The 1975-81 EFE system is used to reduce engine warm-up time, improve driveability, and reduce emissions. The system consists of a vacuum controlled heat valve assembly (EFE valve) and a Thermostatic Vacuum Switch (TVS) installed in the coolant thermostat housing of the engine.

The TVS controls the action of the EFE valve. One port of the TVS is connected to a manifold vacuum source; the other port to the EFE valve. When the engine is cold (as sensed by the TVS), the passage within the TVS is open to allow vacuum to act upon the EFE valve. THe EFE valve closes, which blocks the exhaust flow from the right side cylinder bank. Because of the blockage, the exhaust gas is forced through passages in the right side cylinder head and the intake manifold. The heat from the exhaust warms the carburetor which improves the vaporization characteristics of the fuel. The exhaust gas exits through the left side exhaust pipe until the EFE valve closes. As the coolant temperature increases, the TVS shuts off the vacuum to the EFE valve. The valve then opens, allowing the exhaust gas to follow its normal route through the right side exhaust pipe.

1982 models do not use the EFE system, due to the superior fuel handling capabilities of the Throttle Body (fuel) Injection system.


After the vehicle has sat overnight, open the hood and note the position of the EFE valve control rod. Start the engine and check that the position of the control rod changes (valve closes). If the valve did not close, disconnect the vacuum line from the EFE valve. If vacuum is felt at this line, either the EFE valve is defective or the linkage is binding. If no vacuum is felt, check the condition of both vacuum lines. If the lines are in good condition, the TVS is defective. If the valve does not open as the engine warms, again check for vacuum at the line attached to the EFE valve. If vacuum is present, the TVS is defective; if no vacuum is present, either the EFE valve is defective or the linkage is binding.


Perform this operation only when the engine is cold.

  1. Disconnect the battery cables at the battery.
  3. Raise the front of the vehicle and support it safety with jackstands.
  5. Disconnect the exhaust crossover pipe from the exhaust manifolds and lower the pipe away from the manifolds.

On some models, it may be necessary to completely remove the crossover pipe to allow enough clearance for the EFE valve to slide off the manifold studs.

  1. Disconnect the vacuum line from the EFE valve and remove the valve assembly.
  3. Installation is the reverse of the previous steps. If gaskets are used at the pipe or valve connections, replace the gaskets.
  5. Check for proper operation of the EFE valve.

TVS Switch

  1. Drain the radiator.
  3. Disconnect the vacuum lines from the switch, noting their locations. Remove the switch.

To install:
  1. Apply sealer to the threaded portion of the new switch, and install it, tightening to 15 ft lbs.
  3. Rotate the head of the switch to a position that will permit easy hookup of vacuum hoses. Attach the vacuum hoses to the proper connectors.