A car's thermostat, though small and inexpensive, controls the vehicle's entire cooling system.

Its purpose is to trap coolant in a cold engine. When the engine's temperature rises, the thermostat opens and allows coolant to flow to the radiator. It plays two roles: it supplies hot water for the heater and ensures the engine reaches its proper operating temperature.

The most common types are bellows and pellets.

If the thermostat valve gets stuck in the closed position, coolant is trapped inside the engine and it will overheat. On the other hand, if the valve gets stuck in the open position, the engine warms up very slowly, if at all.

thermostat indicators

You can get an indication of whether the thermostat is working by removing the radiator cap (when the engine is cold) and inserting a thermometer in the coolant.

Start the engine and watch the thermometer. Keep squeezing the upper radiator hose. When the thermostat reaches the temperature at which the thermostat is supposed to open, you should feel coolant surging through the radiator hose.

1. Drain half the coolant from the radiator with the engine cold.

2. Remove bolts holding the thermostat housing. Remove the housing.

3. Lift the thermostat from the housing. Hold the unit to a bright light. The thermostat valve must be closed. You should see no light passing around the valve. If the valve does not close, replace the thermostat.

4. Note the temperature reading on the thermostat. Thermostats are built to be fully open about 20 degrees above rated temperature.

5. Tie the thermostat to the centre of a wooden dowel. Insert a .003-inch feeler gauge between the thermostat valve and housing. Tie a string to the feeler gauge.

6. Place the thermostat in a container of water. The thermostat must be fully immersed, but it should not rest on the bottom or against the sides of the pan. Place a thermometer in the container and heat it.

7. Pull the feeler gauge free when the thermometer shows that water is heated to the temperature stamped on the thermostat housing, + or - 5 degrees. The thermostat valve should be wide open when the water reaches 20 degree above the temperature, give or take 5.

— Paul Messam