How do you know diagnose if thermostat needs to be replaced?

Answer 1

Couple of ways I can think of offhand. If you have a car that you can remove the radiator cap (WHEN THE ENGINE IS COLD) and see the coolant in the top of the radiator is to do the following: Leave the car overnight, or for at least a couple of hours, so it will cool down all the way. Remove the radiator cap, make sure you can see coolant at the top of the radiator. Put your finger in it (REMEMBER ONLY DO THIS WHEN THE ENGINE IS ALL COOLED OFF) and the coolant should feel about 'air temperature'. Then, leaving the radiator cap off, start up your engine. You should not see any significant fluid flow in the radiator, because at this time the thermostat should be closed and no or very little coolant will be flowing through the radiator. Wait while the engine warms up for 4 or 5 minutes...if the thermostat is working correctly, at some point you will see a difference in the water in the radiator...you might see movement, or it might just start to shimmer or look different. Carefully feel the water again...if the thermostat has opened up, the water should start to get warmer as hot water from the engine block is directed through the radiator for cooling. That will give you a general idea if the thermostat is working at all, but not if it is working within specifications. If the thermostat is stuck closed you will never see increased flow through the radiator and the radiator water will stay relatively cool even as the engine starts to overheat. If the thermostat is stuck open you will see coolant flow and start to warm up immediately, even though the engine is not yet up to ideal operating temperatures. Either case is bad. To see if the thermostat is operating at the designed temperature rating, or to pre-test a new thermostat before installing it, all you need is a kitchen stove, a pot with water, and a thermometer that goes up to 212 degrees. Put the thermostat in a pot of cold water on the stove, with the thermometer in the water. Heat the pot of water...as it approaches boiling you should see the thermostat 'open' (a piece of it will move up or down, opening a gap that will allow water to flow through it when inside the engine.) See what the thermometer says the water temperature is when this occurs...compare it to the specs for the thermostat. I think thermostats usually have their designed temperature stamped on them. So if it is a 185 degree thermostat, this should happen someplace between 180 and 190. If it's a 170 degree thermostat, between 165 and 175. you get the idea. I'm saying a 10 degree range is probably ok because I don't know how accurate your thermometer is! Now to get the existing thermostat out for testing can be a bit of a chore depending on the vehicle. Usually it is mounted underneath the fitting where the upper radiator hose goes to the top of the engine or the engine's intake manifold. A couple of small bolts frees up the thermostat cover, you may have to sort of pop it with a mallet to break it loose from it's gasket, and then can take it out to test. You will have to scrape off any old gasket (don't let it fall inside the engine!) and use a new gasket or some gasket sealant when putting everything back together again. Personally, thermostats are cheaper than my labor is worth to myself, so if I was going to all this trouble I would probably just put a new one in, rather than testing and possibly reusing the old one. I DO test new ones on my stove top before starting work on the engine to replace the existing one...I have had a defective-out-of-the-box thermostat, and would rather know it was bad before I put it in the engine. NOTE I am not nor do I pretend to be a qualified mechanic. But I have been working on my own cars for over 40 years, including backyard engine teardowns and rebuilds. So take this advice as worth what you are paying for it (nothing), and plan your work accordingly. Final advice: WORK ON A COLD ENGINE. Don't start opening radiators or removing thermostat housings if the engine is warmed up...you will run the risk of scalding yourself or worse! Good luck, hope this helps, Paul H.