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Derrick, ASE Certified Technician
Category: Car
Satisfied Customers: 923
Experience:  A.S.-Automotive Analysis A6-Electrical A8/L1-Engine Performance Cert., CA Chrysler/Jeep/Dodge Tech
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2001 Hyundai Elantra: My heat/defroster..fine gets going

Resolved Question:

I have a 2001 Hyundai Elantra. My heat/defroster does not work when the car is in idle, but works just fine once the car gets going. This poses a problem when my car is covered in ice in the morning and I can't defrost it! Also, once I've been driving for a while and the heater is going full blast (it's cold here!) and my windshield is finally all defrosted, if I come to a stoplight or something, I can feel the air go from hot to cold. Can you diagnose this problem and give me some idea as to whether it's an expensive fix or not? Thank you!
Submitted: 7 years ago.
Category: Car
Expert:  Derrick replied 7 years ago.

Hi there,

The heater requires that there is hot coolant running through the heater core.

I would first check to make sure your car has coolant in it.

If it does, you most likely have a stuck closed thermostat not allowing coolant flow. It is a relatively common problem on these cars.

Fortunately the thermostat is cheap and easy to replace.

Here is how:

Drain the coolant so its level is below thermostat.
2.Remove the inlet fitting and gasket.
3.Remove the thermostat.
4.Immerse thermostat in hot coolant to check proper valve opening temperature. Replace if necessary.
Valve opening temperature : 82 °C(177 °F)
Full opening temperature : 95 °C(205 °F)


I hope this helps! Let me know if you have any more questions!

Customer: replied 7 years ago.

But if it was a thermostat/coolant problem, wouldn't that mean that I would have no heat at all? Rather than just when the car slows down? Again, my heater works great while the car is moving, but goes from warm to cold air when the car goes from drive to idle, and the defroster doesn't work at all when I first turn the car on. Could it have something to do with the blower fan or something like that?


I don't mean to sound like I'm second-guessing you, but I just don't want to be expecting a simple, inexpensive fix and then get socked with a huge bill...which has been known to happen to me or more than one occasion!

Expert:  Derrick replied 7 years ago.

I understand your concern.


It is most likely a cooling system problem, whether it is a circulation problem, low on coolant or something like that.


It would be best to start be looking into the most likely and cheapest things, before attempting to look into other possibilities.


Let me know what you find!


Thank you,


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