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  1. #1
    Join Date
    Jan 2006
    Sydney, NSW, Australia

    Default "Hi guys I'm back again. I

    "Hi guys I'm back again. I have a LM318 which has behaved normally in the cooling system up until now. The motor has a closed cooling system using normal coolant.

    The boat is on a mooring about 140 miles from home so I only get to it every second weekend religiously.

    On arrival I immediately check fluid levels etc etc. The last two trips I have found the cooling system to be fully pressurised, and when taking the cap off, coolant sprays out over the motor even though it is cold ( ambient temp about 25 degrees ( centigrade ) and two weeks since the boat has been used.

    How can this be ? and why does it occur now and not in the past ? Has anyone had a similar experience or able to offer me some advice.

    Any and all replies will be sincerely appreciated."

  2. #2
    Join Date
    Jun 2005
    , RI, USA

    Default "G'day Raymond, At firs

    "G'day Raymond,

    At first I scratched my head when I read your post. Then I thought about it and realized there are compressed gasses in the cylinders and to a lesser extent in the exhaust. Hopefully that is not what is pressurizing your cooling system.

    I found these notes in a Mercuiser service bulleting online. Seems potenailly applicable to your situation.


    If the cooling system is prssurized, check for a head gasket leak or exhaust gases entering closed cooling system.

    If higher pressure is obtained than listed below, exhaust gases may exist in system.
    Coolant System Pressure
    800 RPM (idle) 6 PSI (41 kPa)
    3800 RPM (WOT) 20 PSI (137 kPa)

    Check the following if excessive pressure or gas bubbles are found.
    a. Head gasket leakage by making a compression test.
    b. Exhaust gases existing in the closed cooling system by using a combustion leak tester.

    NOTE: Snap-on Tools has combustion leak tester available for testing coolant for exhaust gases. (BlockCombustion Leak Tester P/N GA170B)

    Kits are available to measure the pressure at the heat exchanger cap.

    Good Luck,

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