This little device may not appear to be all that important, but it has a big job to do when your engine gets too hot. The threaded end looks a bit like an engine temperature sensor, and that's because it is. It's normally mounted on the intake manifold or engine block where it can sense coolant temperature.
The three ports at the other end are for vacuum connections. One goes to manifold vacuum, another to carburetor vacuum, and the third goes to the vacuum advance on the distributor. At normal engine temperatures, the distributor gets ported vacuum, which is 0 psi at idle. During extended idling on a hot day in bumper to bumper traffic, the engine temperature can start to rise pretty fast, especially with air conditioning on. That's when this little device goes to work. When engine temperatures reach about 225 degrees, this switch changes the distributor vacuum advance source from ported to manifold vacuum, which is high at idle. This advances the timing and engine rpm increases.
The result is increased radiator fan speed and coolant flow through the engine and radiator, which tends to bring the temperature down. Once the sensor determines the coolant temperatures have dropped to a safe level, it reverts back to ported vacuum and the engine timing returns to normal, returning the idle rpm to its normal setting.
Chances are most people have never even noticed when this device is protecting their engine, they just notice that the temperature gauge drops a bit, or the HOT light on the instrument panel extinguishes.
To test, there are two steps to take. First, with the engine running at normal temperature, unplug the manifold vacuum source from the valve and plug the line. Engine rpm should remain steady. If it drops 100 rpm or so, the valve is bad and should be replaced.
If it passes the first test, attach the manifold vacuum line to the valve again and cover the radiator sufficiently to induce a high temperature condition. Keep a close eye on the temperature gauge or warning light. Once the gauge needle rises to the upper limit or the light comes on, you should have noticed an increase in engine idle speed of at least 100 rpm. If not, the valve is bad and should be replaced.
WARNING: Do not allow the engine to overheat while testing. Uncover radiator and raise idle slightly until engine temperature drops before shutting off engine.
Original Ford part number C8AZ-12A091-A or D0AZ-12A091-A