Heating /AC on 2004 Kia Sorento

My heater isn't working properly, sometimes it blows cold air but when you speed up some warm air comes out. I've added coolant but that was a very temporary fix. I can't find where it's leaking in my garage.

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2 answers 2 comments
If you have to add coolant and the level keeps going down you probably have a bad head gasket, should have a shop pressure test the system to figure out why its using coolant
How expensive is it to repair the head gasket connected with my heater?
$1044 to $1400 depending on which shop does the work, but until a shop determines what is wrong don't have the head gaskets replaced go to a Napa autocare center and have them trouble shoot the cause of the coolant loss http://www.napalocator.com/?st=ACMEC
I know this response is late, but sure it will help others with same problem. I also had this EXACT scenario when we bought our used 2004 KIA Sorento, in 2008. Living in the Seattle area (at the time) it was always wet, so finding the "leak" was not easy. I knew it was not in the engine block because there was no water present in the oil. I introduced die (food coloring) into the cooling system, and traced the leak back to the reservoir. The factory unit is thin and the support veins (on bottom) expand and retract with the changes in temperature, therefore causing it to crack and leak over time. I purchased a new one from a KIA dealer and immediately noticed the changes that were made to correct this common issue (way more solid!). So, if you only notice the heat works when accelerating, this may be your problem. For me, there was not enough coolant loss to trigger "over heating", so narrowing the problem took some time. So before you react to "head gasket issues", start with the simpler causes. Not to mention- much cheaper. I think the part was around $90- $130. Good luck!