The 1966 Ford Mustang features a poppet-type thermostat between the radiator and the engine block. If the engine overheats or takes an unusually long period of time to warm up, the thermostat might be the cause of the problem. Fortunately, the Mustang's thermostat is easy to access, as it is at the front of the intake manifold. Replacement thermostats are readily available and the entire replacement process can usually be performed in under an hour.
Remove the radiator cap from the top of the radiator, then partially drain the radiator by removing the bolt on the bottom of the radiator with a wrench. The radiator need only be drained until the level within the radiator falls below the point where the upper radiator hose attaches to the radiator. Observe the fluid level through the radiator cap opening and tighten the bolt back into the radiator when the fluid drops below the upper radiator hose.
Slacken the clamp that holds the upper radiator hose against the thermostat housing with a screwdriver, then remove the hose.
Undo and remove the bolts which secure the thermostat housing to the front of the intake manifold with a wrench, then pull the housing off of the manifold.
Pull the thermostat out of the intake manifold.
Insert the replacement thermostat into the intake manifold. The large spring on one side of the thermostat must face the engine.
Remove the old gasket from the thermostat housing, then coat a new gasket with water resistant sealer and position the gasket onto the housing.
Position the thermostat housing against the intake manifold and install its two bolts with a wrench.
Push the radiator hose onto the thermostat housing and tighten the clamp with a screwdriver.
Pour a 50/50 mixture of new antifreeze and water into the radiator through the radiator cap opening until the radiator is full, then install the radiator cap.
