The thermostat in an automobile measures the temperature of the coolant with a mercury-filled valve. As the temperature gets hot, a spring in the thermostat compresses and lowers the thermostat housing. This increases the flow of fluid through the engine. Over time, the thermostat clogs with coolant sludge. This sludge clogs the jiggle valve and corrodes the spring. Both issues could cause a perfectly good thermostat to stop working. Repair the thermostat, and avoid purchasing a new one.
Remove the thermostat from the engine. In most cases, the thermostat rests inside the water outlet housing. Trace the upper radiator hose to the engine to locate the housing; remove the two bolts that hold the housing to the engine, and lift the housing off the engine to expose the thermostat. Pull the thermostat out of the engine.
Inspect the mercury valve, located inside of the spring. If the valve shows signs of cracking or leaking, you cannot repair the thermostat. Mercury is highly toxic to people. If the valve is in working condition, move to the next step.
Mix a solution of vinegar and water in a 50/50 solution. Clean the thermostat thoroughly with the solution and a toothbrush. Make sure the jiggle valve is clean, as well as the spring.
Place the thermostat into a pot of boiling water. Rest the thermostat on the end with the spring. Place a cooling thermometer in the pot with the thermostat. Boil to 82.2 degrees Celsius. If the spring on the thermostat compresses, you repaired the thermostat by cleaning it. If it does not, discard the thermostat; you cannot repair it.
Install the thermostat back into the engine. Remove the old thermostat gasket; clean the matting surfaces of the engine and the housing, and place a new gasket onto the engine. Place the thermostat back into the engine, and place the housing back on the engine. Bolt the housing in place, using a socket and ratchet.