See Figures 1 and 2
The 6-cylinder and 1975 Mark IV (big block) V8 EFE systems consist of an EFE valve mounted at the flange of the exhaust manifold, and actuator, a thermal vacuum switch (TVS) and a vacuum solenoid. The TVS is located on the right hand side of the engine forward of the oil pressure switch on 6-cylinder engines and directly above the oil filter on the Mark IV V8. The TVS is normally closed and sensitive to oil temperature.
The small block V8 and 1976 and later Mark IV EFE system 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 or oil 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.