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Thread: coolant levels goes down

  1. #1
    Join Date
    May 2008
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    Illinois
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    '06 530xi '11 535i

    coolant levels goes down

    Once its warm out, every time I turn the engine off, it says 'coolant level'. Then I take the cap off water comes up to its normal level. If I can take it off fast enough I can even see the sensor, water levels can go all the way down to the sensor hence the message on the dash. I can't figure where it goes. Where can that much coolant go?

  2. #2
    Join Date
    Jul 2005
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    Sydney, Australia
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    BMW 530i 1993 E34
    Tried topping it up? Sounds like you have a big air bubble circulating around there...

    If you keep having to top it up, you're losing water somewhere... I had a similar problem recently, turned out I was losing it via the heater tap pump (just next to the fusebox).

  3. #3
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    1986 535i, 2012 X5 50i
    I was told that this is do to the vent line being plugged. This is the one coming all the way from the top of the radiator. Disconnect it on both ends and blow it with compressed air. Apparently the hole on the neck of the coolant expansion tank is relatively small and gets plugged very easily.
    Jimmy Moreno
    BMWCCA 115551
    1986 535i, Artic Blue
    2000 740il, Schwarz II

  4. #4
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    E30 S50B30
    You can try squeeze the hoses too if you don't have handy access to compressed air!

  5. #5
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    '06 530xi '11 535i
    Thanks for the replies.

    I don't loose any coolant, It just disappears and comes back once the cap is off. So trapped air makes sense to me, but how can I get rid of it. I read somewhere that putting the front on jacks makes it easier to get all air out of the system easier while bleeding, is that true, has anyone done that?

    I don't recall a vent hose on my expansion tank, my tank is mounted on the side of the radiator.

  6. #6
    Join Date
    Apr 2008
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    Burlington, VT
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    1995 BMW 525i
    I would bet if you look closely it may be leaking from the seal at the bottom of the expansion tank. It drips in the cross member right under the radiator so you won't see any spots on the ground. Take a close look in there.
    1995 Oriental Blau-Parchment-Wood Trim BMW 525i - 5 Speed, Short Shifted & JC Chipped


  7. #7
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    But I never loose coolant, never needed to top off the system.

  8. #8
    Join Date
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    BMW 530i 1993 E34
    I found a leak in my system once by filling a plastic coke bottle full of water (or coolant!), and inverting it into the expansion tank (with the cap off, of course) and then squeezing.

    It applies just enough pressure to see what's going on, and the coke bottle neck fits perfectly in the filler hole to create a good seal!

    Give it a go!

  9. #9
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    Milford MA/UMass Lowell
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    1999 M3, 2002 530i Sport
    Quote Originally Posted by EonPeon View Post
    I found a leak in my system once by filling a plastic coke bottle full of water (or coolant!), and inverting it into the expansion tank (with the cap off, of course) and then squeezing.

    It applies just enough pressure to see what's going on, and the coke bottle neck fits perfectly in the filler hole to create a good seal!

    Give it a go!

    Or you can get a cooling system tester. Not trying to be mean

  10. #10
    Join Date
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    Atlanta
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    17 330i Touring M-sport
    These systems run in a pressurized environment. Once you open the cap, you release the pressure and the level rises. If you can see the sensor when you pop the cap, your level is low. There is a mark on the tank that says "cold". Wait until the car is cold, then fill the tank to that line. If this is a new problem, then you've lost coolant somewhere....
    Last edited by TouringDan; 06-17-2009 at 10:07 AM.


  11. #11
    Join Date
    Nov 2006
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    Portland, OR - USA
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    e34, e39
    Ugh, I'm going to be going through this soon. Our coolant level light popped on today. I grabbed the hose and topped it up so I could get to work for my staff meeting, but soon I'll be looking for leaks. Time to change a few hoses too I think and do a flush.

    Any other suggestions on common places to look for leaks? Thanks!
    No current BMWs.
    1994 Roadmaster Wagon
    1974 Alfa Romeo Berlina

  12. #12
    Join Date
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    Common leak points are:
    1. The expansion bottle - at the bottom, where it mounts to the radiator
    2. Upper and lower radiator hoses
    3. Spider-hose going from the firewall to the radiator and the engine block's water-output (under the Vanos solenoid)
    4. Firewall water inlets/outlets for climate control
    5. Heater core. If you smell coolant while driving, this is probably the culprit.


  13. #13
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    #5 is fun!

  14. #14
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    Nope, no coolant smell. I know that one unfortunately. Which is why I want to replace these hoses now before I have trouble. I'm looking to do them all at once.

    Anything else i should replace at the same time? Aux water pump? Heater valves? I suppose that's all cheap insurance. Or not so cheap.
    No current BMWs.
    1994 Roadmaster Wagon
    1974 Alfa Romeo Berlina

  15. #15
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    17 330i Touring M-sport
    No reason to replace the aux pump or valves unless they are malfunctioning. Tstat and water pump are always wise preventative maintenance.


  16. #16
    Join Date
    Jun 2009
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    san jose, ca
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    BMW 525i, Datsun 510
    had the same problem. turned out to be a small drip from the upper rad hose.
    top it off completely. go drive. check. repeat.

  17. #17
    Join Date
    Jun 2007
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    Santa Rosa, CA, USA
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    36
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    1994 525iA
    Since the symptoms mckazok are very similar to mine I will add a few more details:

    1. I never lose coolant, yet after driving for an hour +, the coolant level sensor will go off when I turn the car off.
    2. When I let the car cool down, I open the cap, and the bleed screw, and the coolant rises back up to the cold line.
    3. Even waiting overnight, the system is pressurized and the coolant level does not rise until I open both the cap and bleed screw.
    4. Again, no coolant is lost. Coolant rises to the cold line when bleed screw and cap are removed.

    I just replaced my radiator because the expansion tank was leaking but kept the old cap, could this be the culprit?

  18. #18
    Join Date
    Jan 2014
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    Triangle, NC
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    1994 318i
    Hi,
    Just checking to see if either saivior #17 or mckazok #1 ever figured out what the cause/fix to their problem...Mine is exactly the same issues. E36 M3
    Zero coolant loss but shows low coolant until the expansion cap is cracked. Once cracked the coolant fills exactly to the proper level. Something is causing a bit of pressure into the coolant system forcing coolant out of the tank. Once the cap is cracked the coolant refills the tank to the proper level.
    Checked head gasket and that's not it.
    Any help would be most appreciated.

  19. #19
    Join Date
    Jan 2014
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    Triangle, NC
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    1994 318i
    Hi,
    Just checking to see if either saivior #17 or mckazok #1 ever figured out what the cause/fix to their problem...Mine is exactly the same issues. E36 M3
    Zero coolant loss but shows low coolant until the expansion cap is cracked. Once cracked the coolant fills exactly to the proper level. Something is causing a bit of pressure into the coolant system forcing coolant out of the tank. Once the cap is cracked the coolant refills the tank to the proper level.
    Checked head gasket and that's not it.
    Any help would be most appreciated.

    - - - Updated - - -

    Hi,<br>
    Just checking to see if either saivior #17 or mckazok #1 ever figured out what the cause/fix to their problem...Mine is exactly the same issues. E36 M3<br>
    Zero coolant loss but shows low coolant until the expansion cap is cracked. Once cracked the coolant fills exactly to the proper level. Something is causing a bit of pressure into the coolant system forcing coolant out of the tank. Once the cap is cracked the coolant refills the tank to the proper level.<br>
    Checked head gasket and that's not it. <br>
    Any help would be most appreciated.

  20. #20
    Join Date
    Mar 2020
    Location
    California
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    My Cars
    E34 540ia
    Maybe put some UV dye and double check for leaks from top and also from bottom of car with UV light. Expansion tank is good place to start, they get hairline cracks overtime that leaks, and need replacing, also radiator cap needs replacing over time.

    Also check sensor, they can also go bad, I am on my third one.

    If you are not losing coolant and think it is a bleeding issue, might be the thermostat was installed incorrectly, there is a hole on top, if it was install upside down with hole on bottom, this will cause air to be trap in your coolant system causing this constant irregular pressure.

    Watch your temperature gauge, make sure you don’t over heat engine when you are driving.

    Good Luck.
    Last edited by E34 Lives; 07-03-2022 at 03:09 PM.

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