Q: Why heater cools off at low idle but resumes to hot setting upon acceleration?

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At low idle heater cools off even at max hot setting but either reving motor or accelerating to any speed will bring temperature up. Once at idle, like at stop light, temp cools down. Could it be the thermostat?

My car has 36000 miles.
My car has an automatic transmission.

Sometimes when a heater core starts to get clogged from dirt and debris inside coolant cores it take higher flow to get good heat out of it. Try to have heater and cooling system flushed and the thermostat replaced to see if it helps increase flow through the heater.

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It sounds like you may have a thermostat that is not maintaining the proper temperature at idle speeds causing the coolant to be circulated at the wrong times. When this happens, the coolant may not be maintained at the correct temperature resulting in temperatures dropping as it passes through the heater core which causes cooler air to be blown into the cab of the vehicle. I would recommend having a professional from YourMechanic come to your location to diagnose and inspect your heating system.

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