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why does my car heater blows warm and then cold

i have a 2002 honda civic 1.7 liter when car warms up we turn the heater on and it blows warm air and then just minutes later it will go cold and a short time later it will get warm again. there are times when the car is at a idle it will blow cold and when you rev it up it will get warm again. i checked all fluids and they are full and also changed heater contol.the car does not seem to be overheating. any answers

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2 Answers from these members:
bikerbigpapa bikerbigpapa
Either you have an air pocket that is trapped in the system or a water pump that is unable to move water through the heater core, blockage or a problem with the actual pump. Doubt there is a problem with the pump as you say it isn't over heating. When the temp goes cold check the heat of the inlet heater hose and compare it to the return hose, They should be within 10 deg. of each other. If not there is an obstruction with the heater core, air pocket. If both hoses are same temp then start checking for an issue with the temp blend door.
by bikerbigpapa bikerbigpapa Earned 42 community points in Automotive
November 21st, 2011
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