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Thread: E46 mystery coolant leak

  1. #1
    Join Date
    Oct 2018
    Location
    St. Louis, MO
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    9
    My Cars
    2006 BMW 325ci

    Question E46 mystery coolant leak

    My coolant is leaking somewhere very slowly over time. There is no dripping and no wetness after inspecting the splash guard. A few months ago I replaced the expansion tank, thermostat, and oil cooler thermostat. I did not check the coolant level after the procedure was complete (also refilled and bled the system BTW) and the low level indicator light never comes on. I checked it out of curiosity a few weeks ago and noticed the issue. The coolant is at the minimum level. The level bobber floats just barely and bottoms out with a very gentle tap. I usually need to add about 100ml to get the maximum level every week and the level then returns to the low state. I recently replaced the oil housing gasket and vanos line. When doing so I inspected all the hoses, clamps, around the thermostat, expansion tank etc. and noticed no crusty residue. The oil also looks very clean even after 8k miles. I have no idea where the extra coolant is going. Any ideas?

  2. #2
    Join Date
    Dec 2013
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    Chicagoland area
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    4,372
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    EXOTICS
    Perform an engine cylinder leakdown test, I think that should solve the mystery!

  3. #3
    Join Date
    Aug 2009
    Location
    Silver Spring, MD
    Posts
    130
    My Cars
    03M3 17M540 18A5SB
    Coolant. Leak.

  4. #4
    Join Date
    Jan 2017
    Location
    USA
    Posts
    284
    My Cars
    E21, E46
    Get a coolant leak pressure tester. Test the coolant system for leaks.

  5. #5
    Join Date
    Oct 2018
    Location
    St. Louis, MO
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    9
    My Cars
    2006 BMW 325ci
    Thanks for the responses. Oddly enough..I checked the level again after a few weeks and the level has remained at max now?? It has been getting in the 40s here if that make any difference..I dont know. Regarding the leak down test..I guess in that case if there is a leak that would most likely indicate a head gasket? If the leak continues I probably will try the $36 coolant pressure tester first and see what happens..

  6. #6
    Join Date
    Jul 2011
    Location
    Arlington, VA
    Posts
    9,933
    My Cars
    2018 BMW M240i
    Head gasket leaks don’t come and go. How full are you filling the expansion tank? Filling the tank so that the red float bob is pressing up against the tank is a no-no. What’s the pressure rating of the expansion tank cap? The factory cap is rated at 2 bar (~29psi). I used to run a 1.4 bar (~20psi) cap on my departed 2001 330Ci.

  7. #7
    Join Date
    Oct 2018
    Location
    St. Louis, MO
    Posts
    9
    My Cars
    2006 BMW 325ci
    Believe it or not I think it has something to do with the cap itself. I have not been overfilling. The bobber does not "top out" when tapped. The last couple of times I have checked..the level has remained stable. I did torque the cap down quite a bit these times.

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