This system is used on 1975 and later light duty models. The six cylinder system consists of an EFE valve mount at the flange of the exhaust manifold, an actuator, a thermal vacuum switch (TVS), and a vacuum solenoid. The TVS is on the right side of the engine forward of the oil pressure switch. The TVS is normally closed and sensitive to oil temperature.
The V6 and V8 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 (V6/V8) or oil (six cylinder) 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.