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Thread: 1985 K100RS coolant boiling over?

  1. #1

    1985 K100RS coolant boiling over?

    I fired my bike up yesterday after about a month of off and on work. I had previously flushed the cooling system with deionized water and refilled with Yamacool. All hoses were replaced including the hose to overflow tank and a new rad cap just because. This was not part of the work I had been doing. Anyways, I noticed a brief coolant leak that seemed to happen as the bike was getting hot and it wasn't from the weep hole. It did it again today. It's coming from the rad cap which is tight. I'm not getting an overheat light and the cooling fan does cycle off and on. My cooling system service did not include a new thermostat, but the bike wasn't doing this before. It would appear that it has started after having sat for all this time. Is it possible that I have now somehow ended up with a malfunctioning thermostat?

  2. #2
    Quote Originally Posted by mcmlcccvrs View Post
    I fired my bike up yesterday after about a month of off and on work. I had previously flushed the cooling system with deionized water and refilled with Yamacool. All hoses were replaced including the hose to overflow tank and a new rad cap just because. This was not part of the work I had been doing. Anyways, I noticed a brief coolant leak that seemed to happen as the bike was getting hot and it wasn't from the weep hole. It did it again today. It's coming from the rad cap which is tight. I'm not getting an overheat light and the cooling fan does cycle off and on. My cooling system service did not include a new thermostat, but the bike wasn't doing this before. It would appear that it has started after having sat for all this time. Is it possible that I have now somehow ended up with a malfunctioning thermostat?
    I think the radiator cap is not holding pressure - new or not. Recheck that it tight and that it is the correct cap.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
    http://web.bigbend.net/~glaves/

  3. #3
    Quote Originally Posted by PGlaves View Post
    I think the radiator cap is not holding pressure - new or not. Recheck that it tight and that it is the correct cap.


    I have no doubt that the cap is definitely tight. I'm going to put the old one back on and see what happens.

  4. #4
    Quote Originally Posted by mcmlcccvrs View Post
    I have no doubt that the cap is definitely tight. I'm going to put the old one back on and see what happens.
    Good idea. That should rule the cap in, or out, as the cause.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
    http://web.bigbend.net/~glaves/

  5. #5
    Quote Originally Posted by PGlaves View Post
    Good idea. That should rule the cap in, or out, as the cause.

    Alrighty. I put the old one back on. As the bike was getting hot, and before the cooling fan switched on, I noticed a little bit of steam coming from under the cap on the frame side (probably because the filler neck is tilted that direction?), but then it does stop. It didn't leak any coolant though, just a little steam coming out from underneath the cap. It is a different design on the underside. The new one, which has the number 1464726 on the top, has a mesh screen on the underside over the center part. The original one, which has the number 1460812 on the top, is open on the underside of the cap in the center. The new one is from Motorworks. Clearly a different design. What gives?

    I wonder if I should replace both of the cap seals on the original unit and just use it? It seems like I should!

  6. #6
    Quote Originally Posted by mcmlcccvrs View Post
    Alrighty. I put the old one back on. As the bike was getting hot, and before the cooling fan switched on, I noticed a little bit of steam coming from under the cap on the frame side (probably because the filler neck is tilted that direction?), but then it does stop. It didn't leak any coolant though, just a little steam coming out from underneath the cap. It is a different design on the underside. The new one, which has the number 1464726 on the top, has a mesh screen on the underside over the center part. The original one, which has the number 1460812 on the top, is open on the underside of the cap in the center. The new one is from Motorworks. Clearly a different design. What gives?

    I wonder if I should replace both of the cap seals on the original unit and just use it? It seems like I should!
    I certainly would not rely on the new cap.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
    http://web.bigbend.net/~glaves/

  7. #7
    3 Red Bricks
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    No steam or coolant should be comming out of the cap.

    1 711 1 464 726 is the correct cap. Correction: 1 711 1 464 720

    1 711 1 460 812 is not the correct cap.

    The complete correct cap is available from your dealer for $27.84.

    New seals are available for your cap, if that is all that is wrong with it.

    17 11 1 460 759 $3.85
    17 11 1 460 316 $5.53 you need one of each.

    I would suggest the complete cap as the seals are not the only thing that goes wrong with the cap. There is the pressure relief function and the function that allows the cap to suck coolant back from the overflow bottle as the coolant in the radiator cools. If either of those don't function correctly, your bike will overheat.




    Last edited by 98lee; 04-13-2020 at 04:26 PM.
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  8. #8
    Typo on my behalf. My new rad cap that leaked coolant, has 1464720 on the cap, not 1464726. The former is the correct number (1 711 1 464 720) according to parts fiche. The original cap has 1460812 on it. Here is a comparison of the old style (my original one) versus the new style, like my new one, from the Euro MotoElectrics web site.

    https://www.euromotoelectrics.com/pr.../radcap720.htm

    Any other thoughts on what could be causing this? Is it possible I have too much coolant in the system? I had previously posted about coolant capacity and followed the advice posted here:

    https://forums.bmwmoa.org/showthread...-clarification

  9. #9
    Quote Originally Posted by mcmlcccvrs View Post
    Typo on my behalf. My new rad cap that leaked coolant, has 1464720 on the cap, not 1464726. The former is the correct number (1 711 1 464 720) according to parts fiche. The original cap has 1460812 on it. Here is a comparison of the old style (my original one) versus the new style, like my new one, from the Euro MotoElectrics web site.

    https://www.euromotoelectrics.com/pr.../radcap720.htm

    Any other thoughts on what could be causing this? Is it possible I have too much coolant in the system? I had previously posted about coolant capacity and followed the advice posted here:

    https://forums.bmwmoa.org/showthread...-clarification
    I don't think excess coolant would cause escape at the radiator cap. The system including the engine case, the hoses, and the radiator are supposed to be 100% full. Then a small amount in the coolant overflow fluctuates as the volume of coolant expands and contracts with temperature. The cap is designed to hold a certain pressure - typically 16 psi but I don't know the BMW specification. There are supposed to be two points of sealing. If an overpressure condition happens due to temperature then the cap should allow coolant to pass and flow to the overflow, but should not vent at the cap. So look carefully at the surface where the cap is supposed to seal and where you are finding it escaping at the cap.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
    http://web.bigbend.net/~glaves/

  10. #10
    3 Red Bricks
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    Quote Originally Posted by mcmlcccvrs View Post
    Typo on my behalf. My new rad cap that leaked coolant, has 1464720 on the cap, not 1464726. The former is the correct number (1 711 1 464 720) according to parts fiche. The original cap has 1460812 on it. Here is a comparison of the old style (my original one) versus the new style, like my new one, from the Euro MotoElectrics web site.

    https://www.euromotoelectrics.com/pr.../radcap720.htm

    Any other thoughts on what could be causing this? Is it possible I have too much coolant in the system? I had previously posted about coolant capacity and followed the advice posted here:

    https://forums.bmwmoa.org/showthread...-clarification
    That'll teach me to use my reading glasses. Yes, 720 is the correct cap.

    As Paul said, check the filler neck for a crack or defect.







    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

    Lee Fulton Forum Moderator
    3 Marakesh Red K75Ss
    Mine, Hers, Spare

  11. #11
    Bluenoser
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    Pretty simple. If it didn't leak before you put the new rad cap on, suspect that it is the rad cap. The old cardinal rule when doing any kind of work on bikes or anything else, if what you did caused an issue, retrace your steps.
    1995 R100Rt with Kenna Sidecar, 1986 K100RT

  12. #12
    Okay. So here is what I did. I removed the filler neck and inspected it. I couldn't see any evidence of damage or it being cracked. I put the cap on it and tightened it as I normally would have as if it was on the bike. I turned it upside down, turned on the sink tap and filled it with water via the rad hose neck. I observed water filling up the threads all around the cap. I concluded that the only way that could happen was the center rubber sealing ring not sealing correctly. I unscrewed the cap, dried everything off and tried it again. The good news is also the bad news-I couldn't duplicate the problem.

    I had a new center rubber gasket so I installed that on the new-style cap, reassembled everything on the bike, topped up the coolant (I had drained close to 1 liter prior to removing the filler neck) and fired it up. As it was getting hot I did not observe any vapors from under the cap whatsoever. I let the bike get hot, cycling the cooling fan 4 or 5 times and it never leaked. At this point I have no reason to suspect that it was anything other than that center rubber gasket.

    Any additional thoughts from you guys as you have way more experience than I do?

    Tom

  13. #13
    GMR 2010 Rally Chair blakduc1's Avatar
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    Get all the air out

    If I remember correctly, when refilling that system with coolant, you have to massage the hoses and keep working it to get all the trapped air out. Then cap it, run it a little and do some more of that until no more bubbles can be forced up by squeezing the hose(s). Paul, does this sound right to you? May not be the problem, but probably a good idea to do this anyway.

    dp

  14. #14
    Quote Originally Posted by blakduc1 View Post
    If I remember correctly, when refilling that system with coolant, you have to massage the hoses and keep working it to get all the trapped air out. Then cap it, run it a little and do some more of that until no more bubbles can be forced up by squeezing the hose(s). Paul, does this sound right to you? May not be the problem, but probably a good idea to do this anyway.

    dp
    Yes, I always massaged the coolant hoses to expel air until I thought the system was full. Then after the first real ride I would wait until the system was cool and then open the cap to ensure the radiator was full to the top. If not I would add coolant directly to the radiator and check again after the next ride.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
    http://web.bigbend.net/~glaves/

  15. #15
    So I still have the problem. I wasn't riding the bike yesterday, and as I've put 600 miles on the bike now, thought I'd check. The coolant has been getting out from the underside of the cap and then into the threads of the filler neck and cap. I massaged the hoses for about 15 minutes and saw a few tiny bubbles come up and the coolant level drop a bit, but still visible in the neck. It has a new cap and a new seal on that cap as a result of the original problem. What do I do now?

    I can't really run the bike until it is hot with the cap off as after a very brief period the coolant rises in the filler neck and will come out the top long before the coolant is even warm.

    NZptOx.jpg

    hS38E5.jpg

    And on the picture below you can see where there was a tiny drip by the bolt head.

    1PyjS2.jpg

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