Q: My car overheated this morning, completely drained the coolant and replaced but still fluctuating to half and below half

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My car overheated this morning, we let it cool completely and drained the coolant and replaced it with coolant and some Lucas super coolant but the temperature is still fluctuating to half and below half when we drive. Since we changed it, it has not overheated but I don't understand why it's fluctuating. Could it be a faulty thermostat?

My car has 162468 miles.
My car has an automatic transmission.

If the fluctuations in temperature are relatively rapid, that is not typical of a failing thermostat. It’s possible there is a temperature sensor fault, wiring issue or gauge problem that makes it appear as if the engine temperature is fluctuating but in fact it isn’t. Having said that, if the thermostat is original, and you’ve had cooling system "issues", it’s definitely wise to replace the thermostat as part of the diagnostic. The key thing of note is thermostats do not last forever and, indeed, when water pumps are changed out, unless the thermostat happens to be new, it should be changed as well. Fluctuating temperatures, or overheating, can be caused by a loose water pump drive belt, a defective water pump, and trapped air in the system so you want to be sure all air is purged. Finally, if the electric cooling fans are working erratically or there are problems with engine tuning and engine management systems, that can cause temperature fluctuations. I think your best bet, because you started off with overheating and are indicating that the cooling system is still not operating stably, is to request an overheating diagnostic. The responding certified mechanic will consider all the possibilities, including temperature sensor and gauge malfunctions, and the mechanic will also advise you as to the wisdom of changing out the thermostat.

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