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Q: How do I know if I need to replace the thermostat on my car? on 1991 Lexus LS400

My cars water level is normal with no change, and I have not observed any leaks, but my car has stated to run warmer than normal. I turned on the heater to avoid it over-heating. I have not allowed it to go into the red on the temp gauge I just run the heater to keep it low. It doesn't matter what kind of driving I do, it now always warms quickly and I run the heater to keep it from getting too hot.
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An auto repair shop would use a non contact temperature gun to measure the engine's running temperature to see if the running temperature is correct. The thermostat should open at approximately 195 degrees in your Lexus. Feel the top radiator hose, it will be cool to moderatly warm until the thermostat opens at which time the top and bottom hose at the radiator will both be warm.
Thank you. Would a shop do this test for free? I would prefer to have a shop test it but I have limited funds at this time. (as do many people these days...)
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You'll have to pay a diagnostic fee to most shops since we pay our technicians to perform the work. It's better to know exactly what the vehicle needs rather than throw a dozen parts at it. With a vehicle your age I would expect you to need a thermostat, hoses and possibly a radiator. The plastic tanks become brittle over time and will fail at the worse possible moment. Have you already replaced your timing belt and water pump?
I am only the third owner of this car, and I have only had it for a year. In this time I have only had to replace the tires (so far). It has lots of miles on it so I know I will start having major problems any time now. I have no water/coolant leaks so far. So no, I have not changed my water pump or timing belt. I also really didn't think it would be the radiator or radiator hoses because there are no leaks, this is why I think maybe its the thermostat. I do not know what the previous owners have replaced. I can maybe find out if the one before me has replaced anything... I will try.
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The radiator doesn't have to leak to be defective. Over time the deposits collect in the core and limit the cooling the radiator can accomplish. That's what the IR thermometer is for. I take several readings from the radiator and if I find cold spots then I know that area is clogged.
Thank you very much for all your help! (i did not know that about radiators, good to know!)
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