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Thread: Hot air only when revving engine?

  1. #1
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    Hot air only when revving engine?

    So I've had a bit of a problem, I have a 2001 330ci. The air will only blow hot when reving the engine. I checked the heater control valve and I can see the little plunger inside moving back and forth when I turn the heat on and off. I also took the valve out of the car and left the car running with the heat on and the two hoses were not spewing coolant. When I revved the engine coolant started pouring out. So I cleaned the valve and made sure the plunger moved freely and it seemed fine. I put the valve back in with the car running and heat on. The lower hose was hot and the upper was cold and no heat coming out of the vents. Went in the car revved it up to about 3k a few times and heat started coming out and both upper and lower hoses to the valve were hot. Any ideas?

  2. #2
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    Symptoms describe low coolant level, or air trapped in the cooling system, probably both.
    Another possibility is obstructed radiator coolant passages caused by corrosion,
    or a failing waterpump. Is the vehicle overheating at all?
    First thing in the morning(engine cold), check and fill coolant to proper level,
    followed by a proper bleeding procedure. Should take no more than 2~3 mornings to completely eliminate air in system.
    Remember this- EVERY time you remove a hose or component in the system-AIR is introduced, causing a need for a bleed.
    Post back with your findings.

  3. #3
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    Thermostat and water pump have about 10k miles on them. Fairly new. So you think it's just air in the system? Not the valve? Also I have been seeing tiny little pieces of plastic in my expansion tank. I was thinking of back flushing thinking those little pieces could be blocking coolant to the heater core.

  4. #4
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    I still suspect air, bleeding will confirm.
    Seems like those are bits and pieces broken off a plastic water pump impeller.
    Did the previous waterpump impelled disintegrate? If so, those are the remains.
    Did you install a quality pump, or the cheapest Chinese knockoff on eBay?

  5. #5
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    I had this problem before and replaced the water pump and thermostat completely back flushed the system and the heat worked fine for about 4 months and now I'm having the same problem. The water pump impellers looked perfectly fine, and I bought, in my opinion, a good water pump with metal impellers. When I rev the car the heat will be scalding hot then after a few mins will just be warm. I put the car on ramps and have been trying to bleed it for the past hour

  6. #6
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    I haven't seen any cheap, low quality parts with metal impellers, so you should be fine.
    If still no circulation after a couple of bleed attempts, then I would check for obstructions here:
    Part#3,4,- http://www.realoem.com/bmw/enUS/show...41#17111707817
    Last edited by MIKYZZ4; 07-26-2017 at 06:07 PM.

  7. #7
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    I'll keep bleeding the system for the next couple days and try to see if that helps. If not I'll try looking for any blockages.

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