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  1. #1
    cheatin' piston popper addicted's Avatar
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    mercruiser 3.0 questions

    Guys, I need some advice. I have a late 80's boat with a 3.0 mercruiser. we have had it for a few years. It was never as reliable or powerful as our other outboard powered boats, but we dealt with it.

    Last year it was pissing me off because if the hard starts, stalling when shifting, ect., so much that I havn't put it in the water this season. I did all the usual maintenence, but never checked compression. Last weekend i got the urge to look into it, and checked compression. It is 135, 130, 110, 165 front to rear. I dont know why i did not check it sooner, but it is what it is. That much of a variation should explain alot of the running issues.

    Looking up the comp ratio, it's 9.3:1, so that converts to roughly 137psi. i suspect that there is carbon buildup on the rear causing the high number, but what about #3? what usually goes bad on these, the rings or the valves?

    I have seen other boats on craigslist with the same motor in supposedly working condition for around $1k. is it worth it to tear into this motor, or should I buy another boat, swap the motor and sell off the old stuff? The boat itself is in pretty good shape for it's age, and the budget won't allow for a reman motor. I also know that selling a boat like this without a good motor is going to be tough.

    What do you guys think?

    Thanks!


  2. #2
    superdave's Avatar
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    Craigs list or Ebay for a motor. You can pick them up pretty easily. Too bad its 3.0, I have 2 3.7's for sale myself.

  3. #3
    dhoad's Avatar
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    the hard starts might be a failing fuel pump, at least that is what happened to me on an old 3.0
    mechanical pump if I remember correctly.

  4. #4
    cheatin' piston popper addicted's Avatar
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    I should have mentioned that last year, while trying to get it running right i replace the following

    -starter
    -fuel pump
    -fuel filters
    -plugs
    - Cap, rotor & points
    - Rebuilt carb

    and did a complete tune up and reset the timing.

    superdave, are those 3.7's direct replacements for the 4cyl? how much you looking to get for a long block?

  5. #5
    superdave's Avatar
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    Make sure your negative cable from the battery to the block is good. This will cause the starter to drag and go out as well as weak spark. I'll bet thats your problem

  6. #6
    cheatin' piston popper addicted's Avatar
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    i plan on taking the boat to my shop early next week and checking some things probably pull the motor and pop the head off. maybe a leakdown test. cranking speed was never a problem, so i doubt the cables are that bad, but worth checking none the less. the erratic compression is what bugs me. something's not right in there.

  7. #7
    GTXX's Avatar
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    seems like a worn cam lobe (exhaust) can cause high compression on a cylinder like you have, sounds like you have other symptoms that might warrant checking that too.

  8. #8
    King_Of_Fun's Avatar
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    This doesn't apply to your immediate problems, but may help in the future: Expect an engine that hasn't been run for a while to start flaking carbon off the piston/head/valves and occassionally foul a plug. Also, does this engine have a distributor with a mechanical & vacuum advance? If it has the mechanical advance (springs/weights) the springs may have rusted and lost their strength so the advance is not correct for the rpm... Ron

  9. #9
    cheatin' piston popper addicted's Avatar
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    well, i took a look at it last night briefly. the rockers are pretty loose, so i think i'm going to adjust the valve lash and recheck compression. also checked the block # against the service manual, and apparently it's a 2.5L and not a 3.0 like the valve cover says. . .

  10. #10
    cheatin' piston popper addicted's Avatar
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    some one definitely swapped the original motor with a 2.5

    adjusted valve lash and rechecked compression. it came up a bit but is still all over. decided to pull the motor.

    popped the head and measured the bore and it is definitely a 2.5. looks like the head gasket may have been leaking between 2&3. check out the pics.

    otherwise the bore and piston look good. not nearly as much carbon build up on the pistons or heads as i was expecting. looks like the valve guides are leaking a bit though.

    i saw a motor on craigslist for 250. may pick that up and see how it looks. . .
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