The thermostat's job is relatively simple, but extremely important. It regulates the engine's operating temperature. It does this by restricting the flow of coolant from the engine back to the radiator. The thermostat is usually located in a housing where the upper radiator hose is connected to the engine. The thermostat is a valve that is held shut by spring tension. A wax filled thermal element in the thermostat opens the valve. As the engine begins to warm up and the coolant gets hot, the wax inside the sealed element expands and pushes the thermostat valve open. This occurs at a preset temperature (typically 195 degrees F. or so), which is usually stamped on the thermostat itself. The thermostat should be fully open about 20 degrees F. above the rated temperature.
The opening of the thermostat allows coolant to circulate through the engine and cooling system. As the temperature of the coolant begins to drop, the wax element cools off and contracts allowing the thermostat to partially or fully close. Thus, by cycling open and shut a relatively constant operating temperature is maintained.
The thermostat is pretty simple and rarely causes problems, but when it fails the results can be disastrous. The worst case scenario is when the thermostat sticks shut, which can happen if the wax element has been damaged by previous overheating, corrosion or age. If it sticks shut, it will block the circulation of coolant between the engine and radiator causing the engine to overheat.
If the thermostat fails to close, which can happen if the sensing element binds up, the return spring breaks or a piece of rust or debris jams it open, the constant flow of coolant through the thermostat will prevent the engine from reaching normal operating temperature. This can cause poor driveability in cold weather, a sharp increase in fuel consumption, little or no heater output, and accelerated blowby and ring and cylinder wear.
To check the thermostat, remove the radiator cap and start the engine while it is cold. Looking inside the radiator, you should see no movement of coolant. If you see movement, the thermostat is stuck open or is missing and repairs are required.
After the engine has run for five minutes or so, the upper radiator hose should start to feel hot signaling that the thermostat has opened and the coolant is now circulating through the system. Inside the radiator, you should also see coolant movement. If there is no movement (and the engine starts to overheat), the thermostat is defective and needs to be replaced.
A suspicious thermostat can also be tested by removing it from the engine and dropping it into a bucket of near boiling water. It should be closed when cold, then open once it hits the hot water, then close again after its been removed and allowed to cool. You can use a thermometer to check the exact opening and closing temperature.