Q: heater doesnt blow hot until over 2500 rpm, and in cold weather temp gauge goes all the way to hot indicating overheating.

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The heater doesn't blow warm air until the vehicle gets over 2500 rpm. When the vehicle goes below 2500 rpm the heater blows colder air through the vents. At the same time, upon running the vehicle after first startup of the day, or after sitting for a while in colder temperatures, the temperature gauge will fluctuate back and forth sometimes going all the way to the H which triggers the engine to run at low power until the gauge moves back to the half way point which indicates normal temps. The engine does not show any visible signs of overheating, but i do not take any chances and i stop the vehicle. The needle goes back to the normal eventually, but its happening more frequently.

My car has 77000 miles.
My car has an automatic transmission.

Hi there. In most cases when this issue occurs, it is due to an obstruction inside the heater core. The coolant system flows through the heater core to supply heat. Over time, the heater core will develop sludge and debris that will solidify inside the core. As engine RPM increases, the coolant flows faster, which can break loose the heater core obstruction. However, it’s also possible that this issue is related to an electrical fault. I would recommend having a professional mobile mechanic come to your location and complete a heater is not working inspection so they can determine what is causing your problem.

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