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Heat blows cold air on idle

13K views 9 replies 6 participants last post by  XJman 
#1 ·
My 04 wrx blows cold air on idle but hot air when driving. Please help.
 
#6 ·
This. Your heater core is the last thing to recieve coolant in your system. At idle your water pump is spinning slower and not pumping as much fluid. Once the coolant level drops enough, no heat at idle. Probable causes are a failing rad cap that is not allowing coolant back into the system so it is replaced by air. Or you may have a minute leak under pressure that you can't see and over time you lost enough coolant to not hit the heater core. Either way, check your coolant level first. Easy button
 
#4 ·
You could have air trapped in the system that gets stuck at the heater core, the thermostat opening and closing or the pump making more pressure could move it.

I would burp the system and make sure you are topped off

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#8 ·
Did you burp the system to see if it was an air bubble? Both times I've had that issue in the past air in the system was the culprit. On occasion you will also hear what sounds like water behind the dash if you have air trapped in the core.

If the system has ever been opened, install of a new radiator or water pump for example, if its not bled right you'll have that. My 2012 jeep had weak heat on idle took it in they burped the system and the vents blew fire, always.

It's a very simple place to start. So I always go there before moving into things that cost more than coolant.

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