Early Fuel Evaporation System: Operation
Thermal vacuum switch is a normally closed switch which is sensitive to oil (4.8L) or coolant temperature (all other engines). With engine temperature less than 105°F (40°C), TVS is closed which allows manifold vacuum to actuator and EFE valve. Vacuum pulls diaphragm in actuator, closing EFE valve.
This causes hot exhaust gases to be routed to base of carburetor. When engine temperature is greater than 105°F (40°C), thermal vacuum switch opens. This stops vacuum to actuator. Without vacuum, spring pushes actuator diaphragm to rest position and opens EFE valve.