This system is used on all 1975 and later engines to provide quicker warmups and improved driveability while reducing exhaust emissions.
Six-cylinder EFE systems consist of an EFE valve at the flange of the exhaust manifold, an actuator, a Thermal Vacuum Switch (TVS) and a vacuum solenoid. The TVS is a normally closed switch which is sensitive to oil temperature. It is located on the rightside of the cylinder block, forward of the oil pressure switch. With oil temperature below 150°F, the TVS is closed which energizes the vacuum solenoid and allows manifold vacuum to the actuator valve. The actuator in turn closes the EFE valve which causes hot exhaust gases to be routed up to the base of the carburetor. When oil temperature reaches 150°F, the TVS opens. This deenergizes the vacuum solenoid, blocking vacuum to the actuator. Without vacuum, an internal spring in the actuator opens the EFE valve, terminating carburetor heat.
The V8 EFE system differs by not using a vacuum solenoid and by using a TVS which is mounted in the coolant outlet housing. The TVS is coolant temperature sensitive and directly controls vacuum. With coolant temperature below 180°F, manifold vacuum is applied to the actuator which closes the EFE valve. When coolant temperature reaches 180°F, vacuum is cut off to the actuator and an internal spring opens the EFE valve, ending carburetor heat.