Thermostats – Understanding, Diagnosing and Replacing

We’ve all heard someone somewhere make the statement that their cars overheated or is running cold because the thermostat stuck. Then some complain the thermostat they bought isn’t working right because the cars running to hot or to cold. The thing is there is a bit more to it than that.

I have seen thermostats start to open before or after the temp that’s stamped on them. They are not exactly certified or anything. Its a freaking spring so its going to open “around” a desired temperature. Any two thermostats will never open at the exact same temperature.

So how does it really work

How a thermostat works


The wax material in the center expands pushing the thermostat open but the spring choice determines at what temperature this occurs. The reason a thermostat seems to “fail” causing the car to run too cold is the spring is either damaged or weakened over time. A stuck closed thermostat is usually from the wax filled cylinder becoming damaged.


Also unless your testing it in a pot of boiling water you will never know the exact temperature of the water at the thermostat. The cars temp gauge isn’t very close to where the stat is located at so the water at the sensor is going to be hotter or colder than the actual water the thermostat is sitting in.

A thermostat is considered to be within spec if it :

  • Opens within 3 to 5 degrees of its rated temperature
  • Is fully open at 10 to 15 degrees above its rated temperature


I will add the howto on the stat change here later.


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