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Thread: Lost coolant. Then I find it.

  1. #1
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    Lost coolant. Then I find it.

    After turning car off coolant level drops. Sometimes enough to trigger low coolant level. Open the cap and pressure is released an coolant rises to correct level. Pressure tested and filled with vacuum. No other issues but does anyone else have this happen?


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  2. #2
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    Bad cap.


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  3. #3
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    This can also be a bad reservoir, look inside on the right side, there should be a plastic tube running from the very top to bottom, usually it shatters, if it's not there then you need a new tank.
    Attn. NEWBIES: Use the search feature, 98% has already been discussed.
    Click the search button, select "search single content type", select the "e36 sub forum" specifically, try the "search titles" then try the "search entire posts".

  4. #4
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    Air is in the system, either in the cylinder head or heating loop. As the engine cools off, the air contracts and sucks the coolant out of the expansion tank but as the engine heats back up, the air expands and forces the coolant out. You need to keep bleeding it till the problem goes away or you will have hot spots in the cylinder head until it warps or cracks. These cars are hard to bleed because of the height of the head relative to the expansion tank, bleed the car with the front end jacked up to help the air escape.
    Last edited by gdavid; 03-24-2019 at 10:20 PM.

  5. #5
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    The car is difficult to bleed for those who don't follow instructions in the bentley manual. Bleed normally, but with cabin heat on. Then top off and close the cap. Then go for a drive either immediately or later that day, with cabin heat controls on. Then shut down the car, and the next day in the morning when the car is cold, open the rad cap and fill up to the brim with water. Then close it. You're done.

    If this doesn't expel all the air from the head, you have other mechanical problems in the cooling system.

  6. #6
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    Lost coolant. Then I find it.

    I put the coolant in with a vacuum system. It held pressure without any issue at the time. I think my issue is the coolant cap and or as mentioned reservoir. But might be air as well.


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    Last edited by lisnowboarder; 03-25-2019 at 01:09 AM.

  7. #7
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    Drove the car for couple miles yesterday to get it hot. I didn’t have any overheating issues along the way. Highway/ around town/ hills etc. Once it sat overnight it did the same thing and sucked up the coolant. Remove the cap and it all came back.
    Verified the tank wasn’t cracked. My guess is the cap. All components are new following a turbo build.


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  8. #8
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    Having the exact same issue here. Ordered a new radiator cap and if that doesn't make a difference, I will bleed the system. However, I am not experiencing any overheating or any other symptom that would make me think there is air in the system.
    Current Whips: 10 E90 ///M3 - 04 GMC Sierra CCSB - 19 VW Atlas

    Past BMW's:
    85 325 - 87 325is M-Tech 1 - 91 318is S50 swapped - 91 318i 'vert
    97 M3/2 - 98 M3/4/5 (Alpine/Modena) - 98 M3/2 - 98 M3/4 (Dakar)
    06 330xi - 09 335i

  9. #9
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    Quote Originally Posted by cabin View Post
    Having the exact same issue here. Ordered a new radiator cap and if that doesn't make a difference, I will bleed the system. However, I am not experiencing any overheating or any other symptom that would make me think there is air in the system.
    I’m not alone! What I found yesterday is if I open the cap for fluid to return it all seams to come from the upper rad hose. After it’s full the upper hose feels empty. I’ve bleed the car ever drive an I’m getting clear coolant.


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  10. #10
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    This is exactly what I was experiencing back in January. Zero overheating, no obvious leaks, coolant level dropping a little bit each day, and longer than normal "condensation" coming from the exhaust. The exhaust condensation was not billowing, just looked like normal cold start condensation but it lasted a lot longer than normal. I finally rented a pressure tester from Advance Auto, discovered a couple small leaks at hose clamps, so I tightened them. Drove it and still lost coolant. Finally redid the pressure test and pulled all the spark plugs, bingo, coolant in cylinder 3.

    I pulled the head and found a crack from the coolant port to the valve, exactly where everyone finds them. My motor has 187k on it. Headgasket was pretty gross looking and appeared to be the factory part.

    It sucked to confirm it was the head but I was so relieved that I found the issue. Getting ready to put it back together this weekend.
    Last edited by spidertri; 03-29-2019 at 02:48 PM.
    11 128i Space Gray slicktop
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  11. #11
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    Guess I could try a pressure test with spark plugs removed. I pressure tested previously with no issue. It is new cut ring head gasket so maybe it’s worth investigating.
    To clarify I’m not” missing” any coolant. I just have to open the cap to see it again.


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  12. #12
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    If your clips are properly tightened, your system is bled, the cap is good, the engine pulls normally, you are not overheating, and you don't have to top up water more than once every 3 months, I suggest you leave it alone and just keep on driving.

  13. #13
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    You may have a slow Heater core leak. The level drops inside it and makes the level change as if an air gap especially when you turn on the heat.

  14. #14
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    Quote Originally Posted by gc325is View Post
    You may have a slow Heater core leak. The level drops inside it and makes the level change as if an air gap especially when you turn on the heat.
    I have some dye I can put in. Doubt it’s a leak. I have exact same symptoms if car is run for an hour or 15min. Leaks would exaggerate as run time increased. I have no negative effects as run time increases.

    As another note to this. I’ve started to not open the cap on cold starts. Car starts an coolant light is on for 10-15 seconds then it goes off.


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  15. #15
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    Definatly sending cylinger gas into the coolant system. Chances are when this happens its a old bad hg and typically not a cracked head. 9 out of 10 times theres a cracked head involved, unless these symptoms are alike.
    With cracked heads, the cracks open as the car gets warm and spew more coolant. Tbh. Ive pulled engines apart with no coolant loss and found cracked heads so i always say prepare for the worst that way your prepared or relieved when the time come comes.
    If you had a leak in your coolant system it would force coolant out, not suck air in. The coolant system is pressurised after the first heat cycle. Its supposed to retain pressure thats literally how you know theres no coolant leaks. Headgaskets are a different ballpark with the obvious harsher conditions and changes from runing high rpm at 180lbs and 2000 degrees to sitting still.
    Nobody would recertify these machines after somebody screwed with them without any visibility into what they did.

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  16. #16
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    Lots of possibilities floating around in this thread...

    After replacing the radiator cap and getting a few miles on it, my symptoms seem to have gone away. Checked the coolant level last night and it was sitting right at the cold line, as it should be. I will continue to keep an eye on it, but so far so good.
    Current Whips: 10 E90 ///M3 - 04 GMC Sierra CCSB - 19 VW Atlas

    Past BMW's:
    85 325 - 87 325is M-Tech 1 - 91 318is S50 swapped - 91 318i 'vert
    97 M3/2 - 98 M3/4/5 (Alpine/Modena) - 98 M3/2 - 98 M3/4 (Dakar)
    06 330xi - 09 335i

  17. #17
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    Quote Originally Posted by cabin View Post
    Lots of possibilities floating around in this thread...

    After replacing the radiator cap and getting a few miles on it, my symptoms seem to have gone away. Checked the coolant level last night and it was sitting right at the cold line, as it should be. I will continue to keep an eye on it, but so far so good.
    Thanks for the update. I found last night it’s not pressure or vacuum in the tank. Rather it’s the small hole in between the two rings on the cap. If I open the cap an put a finger that hole the coolant level doesn’t rise. Its like the coolant in contraction can’t equalize because of the cap.


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  18. #18
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    Quote Originally Posted by spyderg0d View Post
    The coolant system is pressurised after the first heat cycle. Its supposed to retain pressure thats literally how you know theres no coolant leaks.
    Do you mean that it is pressurised after the first heat cycle following topup, and when the system has become cold ? This, would not be true. In fact there would be a vacuum in there when cold.

    After the first topup, pressure is normal. Then start the engine and it gets hot, steam builds up in the expansion tank, and some inevitably escape. Then upon shutdown, the steam recondenses back into water, and now the total volume of liquid coolant is slightly lower than what was there before. This additional space is occupied by the same air that was there when it was first topup up - hence the vacuum.

  19. #19
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    Then start the engine and it gets hot, steam builds up in the expansion tank,
    Your coolant shouldn't turn to steam in a sealed pressurized system. The boiling point rises with increased pressure.

    Gave away my BMWs, driving a VW and an Audi now.

  20. #20
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    Quote Originally Posted by jmo69 View Post
    Your coolant shouldn't turn to steam in a sealed pressurized system. The boiling point rises with increased pressure.
    Correct, there is a leak or crack somewhere. If you cannot find it externally... Welll....

  21. #21
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    Quote Originally Posted by jmo69 View Post
    Your coolant shouldn't turn to steam in a sealed pressurized system. The boiling point rises with increased pressure.
    If coolant does not turn to steam, exactly how is pressure in the cooling created ? Just by everything getting hot? lol

    Coolant turns to steam and occupies the space in the expansion tank. The pressure raises the boiling point of the existing liquid coolant, which remains liquid until a yet higher temperature is reached and then it turns to steam. This raises the pressure even further, which raises the boiling point of the remaining liquid coolant even further.

    The engineers designed the system such that all of this remains in balance at the engine's optimal operating temperature under all engine conditions, both WOT and idle. That's where the volume of coolant, thermostat, size of radiator, air pulled in by the clutch and aux fan etc all come into play.

    For anyone who still doesn't get this, think of the zero pressure evan's cooling system. The temperature is the same, but pressure is atmospheric. Why? Because their fluid has a way higher boiling point than water. But its very expensive so BMW still uses water and coolant.

  22. #22
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    Quote Originally Posted by Thomas525 View Post
    If coolant does not turn to steam, exactly how is pressure in the cooling created ? Just by everything getting hot? lol
    lol

    Yes. Things expand when they heat up. Coolant and air are not immune to this.

    Coolant turns to steam and occupies the space in the expansion tank. The pressure raises the boiling point of the existing liquid coolant, which remains liquid until a yet higher temperature is reached and then it turns to steam. This raises the pressure even further, which raises the boiling point of the remaining liquid coolant even further.

    The engineers designed the system such that all of this remains in balance at the engine's optimal operating temperature under all engine conditions, both WOT and idle. That's where the volume of coolant, thermostat, size of radiator, air pulled in by the clutch and aux fan etc all come into play.

    For anyone who still doesn't get this, think of the zero pressure evan's cooling system. The temperature is the same, but pressure is atmospheric. Why? Because their fluid has a way higher boiling point than water. But its very expensive so BMW still uses water and coolant.
    There is very little steam involved with a modern-day engine's cooling system (while there will inevitably be a very small amount, it doesn't have quite the pivotal role you seem to be assigning to it). The cooling system pressure increases due to the (wait for it ... ) expansion of the volume of that coolant as it increases in temperature. Hence, there is an expansion tank with a small pocket of air that can be compressed (liquid cannot be compressed) to handle the expansion of the coolant. The function of that little pocket of air is why it is as important not to overfill the coolant level as it is not to let it run low.

    The cap on the expansion tank holds a specified amount of pressure.




    OP : Your radiator cap has lost its ability to release pressure. Just get a new cap already.
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    If anyone knows, it's this guy who knows literally nothing about everything.
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    Do you even OT? The mean 401k around here is probably about $6, what with all the shiny cars and $1,000 watches and donations to get other members' Volvos running.


  23. #23
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    Quote Originally Posted by Critter7r View Post
    lolYes. Things expand when they heat up. Coolant and air are not immune to this.There is very little steam involved with a modern-day engine's cooling system (while there will inevitably be a very small amount, it doesn't have quite the pivotal role you seem to be assigning to it). The cooling system pressure increases due to the (wait for it ... ) expansion of the volume of that coolant as it increases in temperature. Hence, there is an expansion tank with a small pocket of air that can be compressed (liquid cannot be compressed) to handle the expansion of the coolant. The function of that little pocket of air is why it is as important not to overfill the coolant level as it is not to let it run low.
    It has been awhile since I last spoke to an someone who believed he was educated when he clearly isn't, so I will try to be polite.Liquids, especially water based ones, HARDLY expand when heated and before they vapourise. The difference in volume is almost negligible. This is basic grade school science.

    Gas pressure in the expansion tank increases greatly, when gas within the expansion tank space increases greatly. And that happens when the coolant vapourises from liquid to steam, which is a gas. Suddenly the room that was occupied by 2 people gets occupied by 4 people - that should be easier to understand.

    The rad cap is rated for 2 bar, which is roughly 2 atmospheres, which is a 100% difference over normal air pressure. Not the 5.8% difference for hotter air, or the insignificant difference for hotter water.

  24. #24
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    Quote Originally Posted by Thomas525 View Post
    It has been awhile since I last spoke to an someone who believed he was educated when he clearly isn't, so I will try to be polite.Liquids, especially water based ones, HARDLY expand when heated and before they vapourise. The difference in volume is almost negligible. This is basic grade school science.

    Gas pressure in the expansion tank increases greatly, when gas within the expansion tank space increases greatly. And that happens when the coolant vapourises from liquid to steam, which is a gas. Suddenly the room that was occupied by 2 people gets occupied by 4 people - that should be easier to understand.

    The rad cap is rated for 2 bar, which is roughly 2 atmospheres, which is a 100% difference over normal air pressure. Not the 5.8% difference for hotter air, or the insignificant difference for hotter water.
    Then tell me why the level in your ecpansion tank is supposed to be checked cold, aka Kalt? I'm sure the level will be higher after a drive, unless it magically grows more until it cools back down.

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    Gave away my BMWs, driving a VW and an Audi now.

  25. #25
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    I have the same issue currently, but I made my own thread as to not hijack yours on a specific question. However, mine overheats at idle if I drive the car hard. Running coolant pressure test now but its not dropping...but then again my thermo is closed so how could it? Anyways if I figure it out I will come back and PM you my findings. Pretty sure its a HG/Head issue BUT want to make sure before I head down that path. GL

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