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  1. #1

    Default "Hi everyone, I've got

    "Hi everyone,

    I've got a Volvo Penta MD7A in my sailboat. When I bought her three years ago the engine had some problems with hot cycling, where it would go from cool to very hot every 2 minutes. I replaced the thermostat and had a radiator shop blow out the cooling manifold, which was completely full of gunk.

    It ran fine for a month, then the current problem started. The engine starts like a champ and runs fine for 15-30 minutes. RPMs at max are 2300. Then the max RPM's slowly creep down. 10 minutes later I can't get more than 1800...in 5 minutes it's 1500, and so on. The bottom end drops out too, and bringing it down to idle will stall it out.

    Here's what else is going on:

    - Temperature is reading cool the whole time.
    - The diesel fuel is two years old, but is crystal clear red and gunk-free.
    - A new racor filter shows no signs of water.
    - If the engine is turned off for 5-10 minutes, the problem resets itself. I.e., back to 2300 max RPMs.
    - Water flow from the back seems a little low.
    - The problem doesn't happen in the slip, even under load.
    - It's not a clogged air vent (still happens with the fuel cap open).
    - I don't think it's a air leak in a line, because it wouldn't run well in the beginning so consistently.
    - I don't think it's a fuel clog for the same reason, and it likely wouldn't die so slowly.
    - The fuel shutoff cable isn't easing itself out from vibration.

    The problem got suddenly worse last week, and now the max RPM's are only 1300!

    My mechanic and I are running out of options, and he's now telling me his next step requires a young priest and an old priest. Any help you could provide would be great!

    Thanks in advance,

  2. #2
    Join Date
    Oct 2008

    Default Have you replaced the secondar

    Have you replaced the secondary fuel filter? You must have even though you didn't say so.
    Have you checked/tightened all the hose clamps and any other connections in the fuel line from the tank all the way to the fuel pump even though you don't think it is an air leak? Have you made sure the o-ring in the fuel filter is seated properly? Do you have an electric fuel pump? Probably not. Having one has many advantages. In this case turning it in as the engine is losing rpm would give you more clues to the problem.

  3. #3

    Default "Thanks Joe. Yep, I've ti

    "Thanks Joe. Yep, I've tightened all the clamps and the secondary filter is clear. I just had the fuel filter replaced but it didn't affect the problem, so I don't think it's the o-ring. And you're right...no electric fuel pump.

    Do you think it could be a cooling issue? It's raw water cooled. The cooling manifold was all full of gunk, but I didn't get to clear out the engine itself. Have you ever heard of something like that cause my kind of problems?"

  4. #4
    Join Date
    Apr 2006
    , Langkawi, Malaysia

    Default "This is often caused by '

    "This is often caused by 'vaporizing' diesel in the system.
    Might be fuel pump capacity (clogged/gunk/membrane).
    Might be fuel blockage in both return and supply.
    Might even be fuel high pressure pump starting to act up when hot.
    Attach a clear plastic hose, make a loop, on the return line and monitor if any air bubbles are building up on top of the loop. If possible let it run into a separate tank to be sure there is no back pressure."

  5. #5

    Default "Please check for small hole/c

    "Please check for small hole/cracking in membrane of the lift pump.this solved the same problume I had. good luck,"

  6. #6
    Join Date
    May 2008
    Kent, the garden of England, UK

    Default If none of the above suggestio

    If none of the above suggestions helps also check the exhaust system for restrictions including the possibility that a rubber hose is delaminating with the inner section collapsing and causing a blockage.

    The cycling temperatute you noted previously may well be due to the water ways in the block being bunged up but i dont think the current problem is related to any cooling problems.

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