The 1975 and later vehicles are equipped with this system to reduce engine-warm-up time, improve driveability, and reduce emissions. On start-up, a vacuum motor acts to close a heat valve in the exhaust manifold which causes exhaust gases to enter the intake manifold heat riser passages. Incoming fuel mixture is then heated and more complete fuel evaporation is provided during warm-up.
The system consists of a Thermal Vacuum Switch, and an Exhaust Heat Valve and actuator. The Thermal Vacuum Switch is located on the coolant outlet housing on V8s, and on the block on in-line 6-cylinder engines. When the engine is cold, the TVS conducts manifold vacuum to the actuator to close the valve. When engine coolant or, on 6-cylinder engines, oil warms up, vacuum is interrupted and the actuator should open the valve.
- With the engine overnight cold, have someone start the engine while you observe the Exhaust Heat Valve (on some V8s, the EFE valve actuator arm is covered by a two-piece metal cover, which must be removed for service). The valve should snap to the closed position.
- Watch the valve as the engine warms up. By the time coolant starts circulating through the radiator (V-type engines) or oil is hot (in-line engines), the valve should snap open.
- If the valve does not close, immediately disconnect the hose at the actuator, and check for vacuum by placing your finger over the end of the hose, or with a vacuum gauge. If there is no vacuum, immediately disconnect the hose leading to the TVS from the manifold at the TVS. If there is vacuum here, but not at the actuator, replace the TVS. If vacuum does not exist at the hose going to the TVS, check that the vacuum hose is free of cracks or breaks and tightly connected at the manifold, and that the manifold port is clear.
- If the valve does not open when the engine coolant or oil warms up, disconnect the hose at the actuator, and check for vacuum by placing your finger over the end of the hose or using a vacuum gauge. If there is vacuum, replace the TVS. If there is no vacuum, replace the actuator.
REMOVAL & INSTALLATION
The Thermo Vacuum Switch (TVS) is located on the engine coolant outlet housing.
On V8 engines, drain coolant until the level is below the coolant outlet housing. No oil need be drained on 6-cylinder engines. Apply sealer to threads on V8 engines. Use no sealer on 6-cylinder engines. Note that the valve must be installed until just snug (120 inch lbs.) and then turned by hand just far enough to line up the fittings for hose connection.