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  1. #1

    Default Yamaha 9.9 4-stroke, water mixing with oil

    Hi there,

    New member, new poster. Looks like a great forum!

    I bought a used 1989 Yamaha 9.9 4 Stroke. After fixing some initial carb. problems, I noticed that oil and water are mixing in the engine. The oil tank is filling up, and I see oil leaking onto the driveway when I test it.

    It holds pressure well (114 and 119 on the two cylinders) at last test. I know that oil and water can mix because of a cracked block, but a solid test seems like it would rule that out. True?

    I also know that a cracked manifold could lead to oil-water mix, and might not affect my pressure reading.

    Anyone know what the most likely culprit is, and maybe the second most likely?


  2. #2

    Default Re: Yamaha 9.9 4-stroke, water mixing with oil

    A 4stroke is expected to have at least 120psi (low) to 210-220psi (new). You must check compression on 4stroke with throttle wide open to get correct readings.
    Water can enter crankcase where there's a gasket around the crankcase as well as seals and orings connnecting any open passage leading to crankcase. Mostly head gasket, oil pan gasket, adaptor plate, etc...If your thermostat is stuck open, an amount of moisture will also accumulates and enters the crankcase overtime or faster with a high degree of corrosion on the block/head.
    Milky oil leaks while parked your boat. It may be the lower unit oil mixed with water. Bad lower units seals!
    Good luck!

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