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Q: Heat gauge is moving around rapidly to hot back to normal

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The truck seems to be running fine no issues I recently flushed the radiator yesterday and added a 50/50 mix thinking it may help the issue , I also turn the heat on when the high seems to rise but the vent produces cold air. It has only gone to where the warning light comes on twice but it will constantly go from a very acceptable level to a close to red no matter if i am going 35 or 70

My car has 274000 miles.
My car has an automatic transmission.

A: When you flushed the radiator, you may have...

When you flushed the radiator, you may have not got enough fluid back into the engine and formed an air pocket. Try and see if the upper and lower radiator hoses are just as hot. If not, then the thermostat is stuck closed or blocked.

Also, when flushing the system the radiator may be partly clogged. If you have a temperature probe then measure the temperature at different parts of the radiator core to see if there are cold spots which indicate a partially clogged radiator. If you need help, have a mechanic come and diagnose your overheating problem for you.

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