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Discussion Starter #1
I have a 2004 B5 1.8t. Early this summer I had to have the heater core replaced. I was driving on the interstate and bam, mist starts coming out the vents. It was a hot day and the air was on. First the dealership replaced a coupling to the heater core. I made it 20 miles and mist started again. Back to the dealer. They then did the heater core. OUCH!
Now, when it is hot out, and I run the air, after about 20 minutes the air starts to not be cold coming out of the vents and the temp starts to rise. If I shut off the air, the temp will go back to normal in a few minutes. So, what is the problem? Radiator? Water pump? Something I can go back to the dealer to get fixed related to their work on the heater core? I've got 103,000 miles and have the original timing belt and water pump I believe. I've only had the car for 3.000 miles and too many dollars!!!
 

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Yes, the thermostat is typically replaced with your water pump (WP) and timing belt. The thermostat is likely the problem with your coolant temps being excessive. Presumably, this is the cause of your high coolant temps.

Good news is: after you've dropped another ~$700-$800 (at an independent shop-dealership will be higher) on the new timing belt, tensioner, tensioner pulley, water pump, t-stat and the serpentine belt(s), the car will be good for another ~80-90K miles.

One last thing, the timing belt needs changing pronto! You're living on borrowed time - timing belt failure will hurt your wallet even more due to the 1.8T being an interference engine (piston/valve contact if that belt or tensioner fails).
 

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Discussion Starter #3
One more thing. If I turn the heat on, moving the temp dial from cool to heat, the temp gauge goes down to normal, 190, faster than if I don't do that.
 

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2004 GLS 1.8T
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One more thing. If I turn the heat on, moving the temp dial from cool to heat, the temp gauge goes down to normal, 190, faster than if I don't do that.
Yeah, that's normal. When air is flowing past the heater core, it pulls heat out of the whole system. How are your radiator fans? There are two, a mechanical one and an electric one. Your symptoms could be caused by the failure of one or both. The electric fan should come on whenever the AC is engaged.
 

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Yeah, that's normal. When air is flowing past the heater core, it pulls heat out of the whole system. How are your radiator fans? There are two, a mechanical one and an electric one. Your symptoms could be caused by the failure of one or both. The electric fan should come on whenever the AC is engaged.
X2. Usually, a failed electrical fan will cycle the ac on and off when idling at a light. However, wouldn't be unusual if the AC persisted and started to overheat the engine (from the increased load, esp at idle.) That would also explain why the air is getting hotter over time. The idea of the condenser is to bring the refrigerant back down to a more ideal compressible temperature to be more efficient. If the refrigerant wasn't condensed down enough, it would reduce the ACs efficiency greatly.
 

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x3 on condenser (electric) cooling fan. Another question though to better diagnose: Does the AC start blowing hot air even when you are driving, at highway speeds, for instance? If it is doing this while sitting in traffic, it is almost certainly the electric fan. If it is doing this even when you are out on the highway it could be a coolant flow problem because the air flow from driving will keep air flowing through the condenser to keep the AC cool.
 

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Discussion Starter #7
doxster22,
It does start blowing hot air even at highway speeds. In fact, it has always been at highway speeds that the AC starts blowing hot air. That has me thinking it is coolant flow. If I turn the temp knob to hot and get air blowing over the heater core, the coolant temp gauge does go down. It makes it pretty darn hot inside the car though.
The electric fan does come on whenever the AC is turned on.
So, poor coolant flow? It's looking like the water pump to me.
 

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A bit of a mystery, because if you had poor coolant flow it wouldn't be running through the heater core very well, either. And, if the electric fan is running, the AC shouldn't be adding much to the heat load on the engine. I'm starting to think thermostat, as hypothesized by Electron Man, above. It's supposed to fail open, but...
 

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Ok. I think with these details that you are right, probably a faulty water pump. The plastic impellor has probably partially broken on the backside.
 

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A bit of a mystery, because if you had poor coolant flow it wouldn't be running through the heater core very well, either. And, if the electric fan is running, the AC shouldn't be adding much to the heat load on the engine. I'm starting to think thermostat, as hypothesized by Electron Man, above. It's supposed to fail open, but...
also a possibility. The only thing is, if the thermostat were stuck closed, it wouldn't take 20 minutes for it to overheat.
 

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Yeah, or cool down with the AC off, most likely - good point. Doesn't really fit anything, but water pump is a better guess than anything else.
 

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Had The Same problem But when i use to not drive my car for say if i was parked and my ac is running like in a couple mins my car heats up but it was a sensor that controls the A/C Fan
 

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still doesn't really explain the AC stopping unless the car is noticing that it is overheating so it disengages the AC clutch.
There is a wire from the instrument cluster to the A/C control relay (in addition to the wire from the ECM). I wouldn't be at all surprised if the cluster, or maybe the ECM, sends a signal to disable the A/C if the engine temperature gets too high.

A thought for the OP: how's your viscous clutch on the mechanical fan? I have the impression that if it fails, that fan won't cool the engine properly at higher RPMs.
 
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