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Thread: Cooling system issue

  1. #1
    Join Date
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    Cooling system issue

    Evening folks! I’ve been having an extremely weird issue after replacing some of the cooling system. Flash back few weeks ago, lower radiator hose springs a leak by the thermostat housing. So being the BMW owner I am, I replaced upper lower hoses as well as expansion tank, hose from water pump to expansion tank, fan clutch and radiator. Closed everything up and bled the system while it was on the ramps. Had several issues getting the lower hose hot, had ok heat so figured I had an air pocket. I finally got the lower hose to feel like it was having coolant pass through it. This whole time, no overheating and gauge right in the middle. Still not feeling confident, I drive the car and all is good. Parked it and came back outside after a few hours and the hoses were collapsed. Slowly opened the cap and hoses expanded and tank was full. So I turned it back on and brought it up to temp. Heat is great, gauge is in the middle but lower hose is cool with nothing going through it and expansion tank is cold.

    It seems I have a few possible issues, low coolant level, bad thermostat and bad cap. Doesn’t make sense that I have no overheating with a bad thermostat. I’m at a loss here. I have zero leaks. I’ve searched, read and have no idea where to start.

  2. #2
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    Sounds like you have some air in the system. Did you turn the heat on and open the bleed screw when you filled it? Maybe try bleeding the system:
    https://www.newtis.info/tisv2/a/en/e...-check/2n6Ib3I
    2000 740i Sport | 2004 330xi | 1988 325i Vert | 2003 Z4 2.5 | 1995 Ford F150 | 2018 GTI

  3. #3
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    Quote Originally Posted by clarkitect View Post
    Sounds like you have some air in the system. Did you turn the heat on and open the bleed screw when you filled it? Maybe try bleeding the system:
    https://www.newtis.info/tisv2/a/en/e...-check/2n6Ib3I
    Yup, I had that screw opened and closed multiple times trying to get it to burp. I’ll keep bleeding it and seeing if that works.

  4. #4
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    The hoses were collapsed? Never heard of that one. Sounds like the radiator cap (expansion tank cap) isn't letting air back in. It also might not be letting air/coolant out as it should when hot, which could have contributed to your leaking hose.

    I agree about double-checking the coolant level. Here's how I like to bleed the system after doing any work on it:
    1) With everything cool, add coolant to the Full mark and have more coolant ready.
    2) Turn the heat on. Open the bleed screw. Leave the cap off. Start the car.
    3) Add more coolant, and keep adding it whenever it's below the Full line.
    4) When the engine is warm enough to open the thermostat, the top radiator hose will get warm and then hot.
    5) Air, bubbles, and then coolant will flow from the bleed opening. When it's just coolant and the bubbles are gone, put in the bleed screw.
    6) Add coolant as needed, again, to the Full mark.
    7) Put the cap on, let it idle longer to build pressure. Check for leaks. Take it for a shot drive and check for leaks again.
    8) When the engine is cold, check for leaks again. Add coolant as needed.

    I like doing it that way because then I know the pump is working, I know the thermostat is opening, and I can tell coolant is flowing through the radiator. (If the top hose gets hot, then coolant is flowing as it should.)

    About the cap... If you wonder whether it's faulty, back it out until it's loose enough to let air in and out. The system won't pressurize, but that won't be a problem unless the car gets real hot. Then you can make sure everything else is working as it should be. (I've done that dozens of times when I've had slow coolant leaks -- they're slower with less pressure. If you're worried about the cap unscrewing you could take it off entirely, though I've never had that happen.)

    And finally, you probably need a new cap.

    - - - Updated - - -

    I started this thread recently, to help folks understand how their radiators and expansion tanks work:

    https://www.bimmerforums.com/forum/s...ding-Explained
    Last edited by R Shaffner; 01-20-2019 at 07:57 AM.

  5. #5
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    Bmw method is to open bleed screw, turn on heat to full and fan on low. Fill until coolant coming out of the bleed screw no longer has bubbles. Run through heat cycle then top off.

    https://www.newtis.info/tisv2/a/en/e...-check/2lizGfU

    On the bright side it sounds like your cooling system is leak free!
    2000 740i Sport | 2004 330xi | 1988 325i Vert | 2003 Z4 2.5 | 1995 Ford F150 | 2018 GTI

  6. #6
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    Now that I think more about it, I wouldn't let that car get hot with the cap on. It might cause more leaks by not releasing the pressure. If I were you, I'd leave the old cap loose until you get a new one.

  7. #7
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    Thanks gentlemen, appreciate the insight. Pisses me off it’s a brand new expansion tank and cap that may be causing issues. I’m a pretty good DIYer, don’t and can’t do everything by this cooking system bleeding is frustrating. In ever other car I’ve done this on, you either get no heat or it overheats when you have a problem, but not this one hahaha

    Several different bleeding methods make it difficult to figure which one will work for you. I’ll go back out today and try the one above and see what happens.

    Thanks again!

  8. #8
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    Don't worry about a collapsed lower hose - that can happen if the system is over-bled when hot. The system is pressurised under normal operating conditions - and if you let the air out when the system is hot, as the coolant contacts the hoses will collapse.

    The trick is to complete the bleeding procedure when the engine is cold - or at least, not at normal operating temperature. It is the pressure in the system that allows the coolant to remain a liquid at 108C - so it is important that you allow the system to create pressure by opening the cap when the system is cold after bleeding - or - you will get collapsed hoses!

    However, it seems like you stumbled in the right direction anyway!
    Timm..2007 E64 650i Individual Sport..1999 E31 840ci Individual Sport..ex owner of 2000 E38 740..1999 E38 740i V8 M62..1998 E38 735i V8..1993 E32 730i V8..1988 E28 518i


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  9. #9
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    Quote Originally Posted by Timm View Post
    Don't worry about a collapsed lower hose - that can happen if the system is over-bled when hot. The system is pressurised under normal operating conditions - and if you let the air out when the system is hot, as the coolant contacts the hoses will collapse.

    The trick is to complete the bleeding procedure when the engine is cold - or at least, not at normal operating temperature. It is the pressure in the system that allows the coolant to remain a liquid at 108C - so it is important that you allow the system to create pressure by opening the cap when the system is cold after bleeding - or - you will get collapsed hoses!

    However, it seems like you stumbled in the right direction anyway!
    Timm -- (I've seen your site and your posts. Nothing but respect and appreciation. Thanks for all of that.) Quick questions...

    In all my years of working with cars, I have never seen a radiator hose collapse. That includes cars that have been properly bleed and not, those that have had caps off when hot and cold, etc. Pretty much every condition, I've seen. Never a collapsed hose.

    Most coolant caps let air back into the system easily, with little pressure difference. Are your saying you've seen collapsed hoses? If so, why isn't the cap just letting air into the the system?

  10. #10
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    Yes, the cap has both pressure relief (at 2.0 Bar) and has a one-way valve that lets air back into the system - but only at a few psi, enough to collapse the hoses in the right conditions (coolant level not correct and the pipes hot when pressure is relieved).
    Timm..2007 E64 650i Individual Sport..1999 E31 840ci Individual Sport..ex owner of 2000 E38 740..1999 E38 740i V8 M62..1998 E38 735i V8..1993 E32 730i V8..1988 E28 518i


    My BMW Repair YouTube Channel
    My Current 840ci Sport Individual
    My Current 650ci Sport Individual
    My E31 Repair and Information Website
    My E38 Repair and Information Website
    My E63/E64 Repair and Information Website

    Chase - Heroes to a generation

  11. #11
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    Thanks for the help guys. Drained, refilled and bled the system again and all is well. It seems I wasn’t letting it bleed long enough to allow the thermostat and heater valve to all function properly, at least that’s what my shop said. They vacuum bled it and let it idle/run at various rpms for a few hrs to ensure they weren’t any issues. Then it say overnight and everything is good to go.

    Now it’s posted for sale

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