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  1. #1
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    Default Marinised 308 Help

    Got myself an older ski boat. Has 308 that had apparently had a top end rebuild. Got it running but it lost oil pressure and the oil has gone milky. Should I start with a comp n leak down test then look at manifold the heads ?

  2. #2
    been here .......too long Smitty2's Avatar
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    change the oil first.. sounds like you have a sump full of water
    then
    pull the plugs and crank it, also check if the plugs are wet, water wet!
    then do the comp and leak down tests

    I suspect a head gasket has gone, sump has water in it
    and the oil pump does NOT pump water
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  3. #3
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    If the oil has gone milky, I assume it was originally not milky. So I don't see any benefit in changing the oil. Comp test is a good idea, least invasive & might give decisive results [ like cyls down on compression ]. If test #s are consistent & don't vary by more than about 10%, then I would next remove the intake manifold.

  4. #4
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    Oil was clean before start up, I wasn’t happy with the blue rtv I was seeing on the head bolts so have pulled the heads planning to throw new gaskets on. I found a problem with one head bolt that goes into the valley missing. When manifold was off it has cracked the thread in the valley. Will they torque down ok. The motor has recently been apart with texts on all pistons so this happened in the past

  5. #5
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    The little bolts are known to break the block in the early ones.

    Is this a closed cooling system with a heat exchanger ?

  6. #6
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    No it's old school pick up style

  7. #7
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    Yep, they have problems with rust; tend to both rust out and gum up with rust and overheat.

    I’d get rid of the milky oil, change the filter and put in fresh oil and do as Smitty suggests and take a careful look at your plugs.

    Also, look under your oil filler cap for rusty residue and check for a corrosion line on the dip stick.

    Then run the compression test.

    Nine out of ten times it will be a blown head gasket but you want to check it isn’t a more long standing issue.

    They can get very gummed up with rust in the coolant passages.

    Let us know what you find

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    I’m having some forum issues so apologies for my post seeming out of sequence

    These blocks are cast iron and are known to gum up when no anti corrosive is present in the coolant in normal vehicle applications; the simple marinisation process exacerbates this by adding more air to promote corrosion.

    When a simple rebuild is done it won’t fix the problem, the block needs to be flushed, hot tanked and rodded and a heat exchange system installed.

    There are other ways it was dealt with in the old days such as flushing the cooling system with pure coolant after each run but even for someone who can be relatively ambivalent about modern developments the pollution they cause is pretty sick making and I would not recommend them.

    I have also seen people use tank liner fluids to coat the coolant passages to prevent rust. This doesn’t seem sensible to me.

    On your rtv issue and cracked thread, GMH recommended that bolts were dipped in sealant. I can remember tubs of black goop sitting on benches that all the through bolts had to be dipped in. They knew bolts would penetrate through to water.

    If you know you have a crack around a thread I wouldn’t attempt to torque it up, that’s just as likely to split it and irreparably damage the casting. Some of these can be helicoiled so that may be a solution, otherwise you can have it welded.

  9. #9
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    The inlet manifolds and pretty much anything alloy corrodes like hell on the old school ski boats.
    Inlet manifold is a common leak point straight into the valley, even the timing cover will go through.

    Corroded alloy exhaust manifolds generally end up with a more serious failure.

    Just work through it methodically and you'll nut it out.

    Find a way to pressure test the cooling system, you may need to make/improvise some parts to do it but it will save you some sanity.

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    was it used in salt or freshwater?

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    As Above anything alum goes first on a raw water system especially if salt water. Cast iron actually goes the distance. Getting the an alum intake and timing cover ceramic coated inside and out is a good idea.
    Last edited by RedTaxi; 04-01-2021 at 06:19 PM.

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