Understanding the purpose of the thermostat is important when diagnosing an overheating problem. Engines, due to the principal of internal combustion, create a high degree of heat. Heat applied to metal tends to expand and contract when it cools. When the engine was designed, these factors had to be taken into consideration.
The first thing in design was to determine what would be the optimal temperature for the engine to operate and what was the total expansion at that temperature. The internal combustion engine is not a very efficient design even in its latest configurations. When an engine is cold all the parts are contracted. The pistons are loose in their bore until they heat and expand to their normal operating temperature. When they reach their target temperature, the pistons and rings will expand sufficiently to seal all the combustion gases in the cylinder with very little passing the rings. Until this temperature is reached, the rings are not tight in their bore and a great deal of combustion gets past the rings and enters the crankcase. This is why an engine does not run as well when cold as when it is at normal operating temperature.
For the above reasons, the manufacturer wants the engine to warm up to operating temperature as rapidly as possible. This keeps the oil cleaner and lowers emissions. The thermostat plays an important role in controlling the engine temperature. The coolant will move from the radiator through the bottom hose on the radiator to the water pump. The water pump pushes the water through the engine's cooling jackets and through a port to the heater core, out of the other side and back to the intake manifold just behind the thermostat housing.
The water must pass through the thermostat to return to the top (and the radiator) to be cooled. The thermostat consists of a thermo spring and a cap with a hole in the center that is closed by the thermo spring. To get the engine to heat up as rapidly as possible, the thermostat is closed and does not allow any water to pass through to the radiator. As the heat builds up in the engine and the water temperature climbs to normal the thermo spring begins to expand and push the thermostat cap open allowing water to return to the radiator.
If the thermostat sticks closed, it will not allow the water and anti freeze to circulate to the radiator to adequately cool. This is where it will cause overheating. As a note any time the water pump or radiator is replaced the thermostat should always be replaced. Also, if the thermostat sticks open, it is possible for the car to overheat. The water will circulate too fast without time to cool.
