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Vehicle: 2005 Mazad3 Hatchback (1.5 L) 4AT

Issue: The heater core unit beneath the dash unit gets very hot after around 15 mins drive (AC is turned off) While the coolant temp. gauge shows not overheating and no significant coolant level drop...

Could this be caused by exhaust manifold leak ? If not, what could be the possible cause?

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    How hot is "very hot". Bear in mind that this is full of coolant so it will typically run between 90 and 110 degrees Celsius. – Steve Matthews Feb 4 '19 at 12:18
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In general, heater cores are supposed to get "hot". You may have defective heater core valve (name varies by manufacturer).

This valve normally blocks off flow to the heater core, but since heat doesn't necessarily need flow from the coolant to transfer, the coolant trapped in the heater core when the valve is closed still gets warm. So, without knowing more details on the exact temperatures we are talking about, it could be normal.

If you find your A/C does not work that well, it may be that that valve has failed open.

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  • Thanks man, the A/C of my car indeed works very inefficiently.. the air will get warm after around 15 mins. I was told by my mechanic that my car has refrigerant leakage from under the dash, no repair has been done, just refill refrigerant every year... Since there's no leakage found from any coolant hose, do you think a reverse flush into the heater core would solve the issue? – hhc761 Feb 8 '19 at 3:24
  • I'm still not convinced there's an issue to solve. If the A/C isn't working well because of that leak, then it may not be the heater core. – Nick Feb 8 '19 at 15:12

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