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  1. #1

    Default LM318 - coolant overflowing

    I spent last season debugging cooling issues with the Twin 318s in my "new" 78 Silverton 31. I replaced a leaking heat exchanger on the port engine and all seems fine there now.
    The starboard engine has me miffed though. It would run at 160 or so at cruising speeds, but run hot at idle speeds. This caused me to watch temps like a hawk. If I didn't catch it as it crept up it could hit 200 degrees. I cleaned out the heat exchanger and replaced the entire raw water pump. This improved things but the system would still push coolant out into the overflow tank and overflow it. I got to the point of running the overflow line from the tank into an antifreeze jug to catch the overflow.
    This spring I've pulled the manifolds and will probably replace them even though they were running cool. While it apart I did a compression check and found pressures between 100 and 125 except for number 8 cylinder, which is 85-90 psi. Granted this is all done cold, after sitting all winter.
    My question is, could the head gasket be leaking compression into a coolant gallery and creating the overflow? The engine runs smooth with full power, and doesn't create any steam thru the exhaust or burnt antifreeze smells.

  2. #2
    Join Date
    Mar 2019
    Location
    Ottawa Canada
    Posts
    440

    Default Re: LM318 - coolant overflowing

    If the head gasket was leaking into the coolant. You would most likely see bubbles in the overflow tank. And wouldn't cause an overheat situation. Is the coolant disappearing other than entering the overflow? You don't need to add coolant?

  3. #3

    Default Re: LM318 - coolant overflowing

    The only time I added coolant was to replace what had been pushed out the overflow. I dont recall seeing bubbles coming into the overflow tank. Would it be obvious or something more subtle?

  4. #4
    Join Date
    Mar 2009
    Location
    Portland, Oregon,
    Posts
    10,727

    Default Re: LM318 - coolant overflowing

    If a head gasket is failing, it may be allowing combustion gasses to enter and pressurize the coolant jackets as well as the E/G side of the heat exchanger. If so, this could explain why an excessive amount of coolant is being pushed out into the recovery reservoir.

    I’d suggest buying some hydrocarbon test strips. The strips will be submerged in the E/G coolant.
    These should indicate if a head gasket is failing, allowing combustion gasses to enter the cooling passages.



    .
    Last edited by RicardoMarine; 04-05-2019 at 08:31 AM.
    Rick ... aka Ricardo..... AQ series Volvo Penta repair
    Portland, Oregon
    28' SDN F/B w/ twin Volvo Penta 5.7L DP drives

    "Intellectual growth should commence at birth and cease only at death." Albert Einstein

    Please... no PMs! Post your questions on the forum.

  5. #5

    Default Re: LM318 - coolant overflowing

    Quote Originally Posted by RicardoMarine View Post
    If a head gasket is failing, it may be allowing combustion gasses to enter and pressurize the coolant jackets as well as the E/G side of the heat exchanger. If so, this could explain why an excessive amount of coolant is being pushed out into the recovery reservoir.

    Id suggest buying some hydrocarbon test strips. The strips will be submerged in the E/G coolant.
    These should indicate if a head gasket is failing, allowing combustion gasses to enter the cooling passages.
    Unfortunately, this is not proactical at this time as the manifolds are off and not able to run the motor. I was planning to do a leak-down test, which as I understand, should reveal the path of leakage on the one cylinder that has bad compression. At that point I will probably just remove the head for gasket replacement.

  6. #6
    Join Date
    Sep 2010
    Location
    Jersey Shore
    Posts
    5,313

    Default Re: LM318 - coolant overflowing

    in addition, I would pull off the circulation pump(The one on the engine) and check the impeller.
    Chris
    Dockside Marine Services
    Jersey Shore.
    Mercruiser/Mercury Certified
    Let the insanity begin.

  7. #7

    Default Re: LM318 - coolant overflowing

    Quote Originally Posted by Docksidemarineservices View Post
    in addition, I would pull off the circulation pump(The one on the engine) and check the impeller.
    Good point. I have the thing half pulled down now. Might as well check it.

  8. #8
    Join Date
    Mar 2009
    Location
    Portland, Oregon,
    Posts
    10,727

    Default Re: LM318 - coolant overflowing

    Quote Originally Posted by swampyankee View Post
    Good point. I have the thing half pulled down now. Might as well check it.
    On the rear side of the Circulating pump, there will be a cover.
    For the cost of one gasket, you can remove the cover and inspect the Metallic impeller.
    If it's an automotive version, it will not be SS or Bronze, and may be rust compromised.

    Click image for larger version. 

Name:	SBC marine version circ pump impeller.jpg 
Views:	19 
Size:	19.1 KB 
ID:	19916


    Rick ... aka Ricardo..... AQ series Volvo Penta repair
    Portland, Oregon
    28' SDN F/B w/ twin Volvo Penta 5.7L DP drives

    "Intellectual growth should commence at birth and cease only at death." Albert Einstein

    Please... no PMs! Post your questions on the forum.

  9. #9

    Default Re: LM318 - coolant overflowing

    RicardoMarine Re: LM318 - coolant overflowing
    Originally Posted by swampyankee
    Good point. I have the thing half pulled down now. Might as well check it.



    On the rear side of the Circulating pump, there will be a cover.
    For the cost of one gasket, you can remove the cover and inspect the Metallic impeller.
    If it's an automotive version, it will not be SS or Bronze, and may be rust compromised.

    Attachment 19916

    Useless information, no such pump or cover exists for a small block Chrysler

  10. #10
    Join Date
    Mar 2009
    Location
    Portland, Oregon,
    Posts
    10,727

    Default Re: LM318 - coolant overflowing

    Quote Originally Posted by mcomm View Post


    Useless information, no such pump or cover exists for a small block Chrysler

    My mistake regarding the cover. I guess that I had the SBC on my mind when I posted.
    None-the-less, the OP can pull the circ pump and inspect the impeller as Chris suggested.


    BTW mcomm, perhaps you could find a way to be a bit more polite and forum friendly.


    .
    Last edited by RicardoMarine; 04-07-2019 at 09:55 AM.
    Rick ... aka Ricardo..... AQ series Volvo Penta repair
    Portland, Oregon
    28' SDN F/B w/ twin Volvo Penta 5.7L DP drives

    "Intellectual growth should commence at birth and cease only at death." Albert Einstein

    Please... no PMs! Post your questions on the forum.

  11. #11

    Default Re: LM318 - coolant overflowing

    A little update -
    After a compression check showed about 20psi less in one cylinder than the rest, I did a leakdown test to see if that cylinder was leaking into the freshwater coolant gallery. It showed an acceptable percent of leakage and no leaking into the coolant. I cross-checked a few other cylinders to see, and they all read the same.
    Now, these tests were done with a cold engine and after having sat all winter. At this point with a non-running engine I don't have any other more accurate diagnostic test I could do. I installed new manifolds and risers on both engines. One interesting thing I observed when pulling the manifolds is, when I was removing them one of the mounting studs at the end of a manifold unscrewed from the head. When it came out coolant poured out of the tapped hole. There is a freeze plug right around the corner so it made sense that the stud hole is drilled into the coolant gallery. The end of the manifold was heavily rusted, and knocking the scale off revealed a good size hole in the manifold (open to the air, not into the cooling lines).
    I wondered maybe that the exhaust pressure was driving in around the stud and up into the coolant gallery, thus pressurizing the freshwater coolant system. With manifolds as rusted and blocked as these were, anything could be possible.
    When I installed the new manifolds I put some RTV sealant on the threads of the studs and screwed them in tight.

    Still alot more work to do in the engine room before launch, so it'll be a while before I find out if the new manifolds and gaskets, etc. made any improvement.

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