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Q: Engine overheating can't figure out why?

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My thermostat is going bad on me so I replaced it. now after about 30 minutes of driving the temperature spikes then Falls. Heater blows cold air. I don't appear to be losing any coolant and a vibration when it overheats
My car has an automatic transmission.

If you did not purge all the air out of the cooling system the temperature would spike and you would loose heat. If all the air is purged and overheats after driving a while then the fan clutch may be causing the overheating if it is not locking the fan to keep the engine cool. Have the fan clutch and water pump checked for looseness or wobbling that can lead to water pump bearing failure.

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