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Thread: 528i, Coolant Not Returning to Overflow Tank

  1. #1
    Join Date
    Dec 2015
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    Plymouth, WI
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    43
    My Cars
    528iT, Fiero, Stingray

    528i, Coolant Not Returning to Overflow Tank

    Hey Everyone,

    2 years ago I did a full cooling system refresh on my 1999 528iT @200K miles. Radiator, hoses, waterpump (metal impeller), thermostat & housing, overflow tank, cap, everything. It was a good refresh kit from ECS, not Chinesium crap from ebay. I've had ZERO issues with the cooling system before or since the system refresh. I did it preventatively so I could start with a solid foundation. The car has been faultless and now has 212K miles on it. I do track days with it, drive it everyday and it has never given me any problems.

    Recently I've noticed that when I check the coolant level, there is a noticeable pressure change when I unscrew the cap. I believe it to be a small amount of vacuum in the tank. The coolant level will be low (the dipstick is ~2" below the top of the housing where it should be) band if I unscrew the bleed screw, the tank will fill up to its normal level! As if coolant is filling up the overflow tube and not returning to the tank. This will be the case even after not having driven the car for a day or two. The bleed screw is NOT an oem plastic one, it is an aluminum unit from ECS. I've compared it to the old plastic one I still had laying around and it is identical in length and also has the grooves along its axis through the threads.

    I thought I found the issue in some plastic flashing on the overflow tank cap that could have prevented the coolant from flowing through the hole that I believe returns the coolant through/past the cap and back to the tank. I cleaned that up but it didn't change anything.

    Has anyone else experienced this?

    Thank you for any help,

    -Tim

  2. #2
    Join Date
    Sep 2006
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    hiss by my window
    This made me curious so I just tried an experiment with my own '99 528i. It has been standing in the garage for several day.
    I opened the cap on the reservoir and the stick was right where it ought to be, approximately level with the opening. Unscrewing the bleed screw I hear gurgling and the reservoir begins to fill. The stick is now well proud of the opening and I assume I've introduced a bit of air into the system.
    I think the condition you're seeing is normal and you are just a bit underfilled.

    If you can leave two black stripes from the exit of one corner to the braking zone of the next, you have enough horsepower. - Mark Donohue

  3. #3
    Join Date
    Sep 2011
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    2002 BMW 530i Sport
    Quote Originally Posted by ross1 View Post
    This made me curious so I just tried an experiment with my own '99 528i. It has been standing in the garage for several day.
    I opened the cap on the reservoir and the stick was right where it ought to be, approximately level with the opening. Unscrewing the bleed screw I hear gurgling and the reservoir begins to fill. The stick is now well proud of the opening and I assume I've introduced a bit of air into the system.
    I think the condition you're seeing is normal and you are just a bit underfilled.
    And just a bit underfilled is exactly where you want to be.

  4. #4
    Join Date
    Jan 2016
    Location
    Ca
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    2001 525it
    I had a tank that was not made correctly so it dint bleed right

  5. #5
    Join Date
    Apr 2008
    Location
    Australia
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    513
    My Cars
    E39 99 528iT
    The hole you can see in the filler neck only allows coolant flow through it when the filler cap is off.
    When the cap is fitted the coolant does not return to the expansion tank via this hole. This why the filler cap has 2 O rings on it.
    The coolant flows into the RHS half of the tank (when standing in front of the car and looking into the engine bay) via a tube the goes down to the bottom of the tank on that side.
    This coolant flow comes from the top of the hot side of the radiator via the tube under the fan shroud.
    It is effectively a continuous coolant bleed that that ensures that any air bubbles trapped in the system eventually end up in the expansion tank.
    The normal level in the expansion tank is approx half full with a cold engine. This is well below the top of the radiator.
    If you open either of the bleed screws, after effectively bleeding the cooling system using the cold rev procedure as specified by BMW, the first thing that happens is air enters the system as the coolant level in the radiator, engine and expansion tank equalise.
    There is a syphon effect happening here which you break when you open the bleed screws.
    There are pics of the expansion tank cut in half on the web - Google is your friend.
    Edit: Spelling corrections
    Last edited by Aussie528iT; 09-26-2020 at 08:02 PM.
    RonR

    99 528iT M52TU 5HP19
    RHD Euro Spec

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