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Mercruiser 5.7L Low RPM's Under Load

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  • Mercruiser 5.7L Low RPM's Under Load

    I have a 1997 Monterey with a Mercruiser 5.7L sterndrive. The boat has routinely cruised at 3200 RPM For years. The other day was no different, took it out and ran it around the lake at 3200 RPM. Later that day I needed to disconnect and reconnect the alternator because the bracket needed to be replaced. When I took the boat out today it only got to 2900 RPM and feels sluggish, not quite getting on plane.

    Playing with the trim can get me a few more RPM but not the problem.

    Looking for suggestions on the potential cause of the low RPM and sluggishness.

    Thanks!

  • #2
    Perhaps tack has become inaccurate. Did you disconnect batteries before removing alternator? Manually connect reliable tack and compare the difference, if any. At least that will be one of the possibilities eliminated. Have you determined the possibility of water in the bilge and causing excessive weight? Is boat stored wet, trailered or open to rain water? Any hidden compartments that water may have accumulated? Do you frequently use boat or is there a possibility gas crap settled and finally was freed to get inside fuel system that feeds motor?

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    • #3
      Tack could be inaccurate but even without tack reading I can hear that RPM's are lower and boat doesn't have as much power. I did disconnect the batteries prior to working on the alternator. No apparent water in the bilge, will check other compartments but boat is stored on a lift under a covered dock when not in use. Boat gets used routinely over the course of the spring, summer and fall. Something could have always broke loose from the fuel tank or could have got some bad fuel from the Marina the other day...One day it was fine, the next morning it wasn't.

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      • #4
        Without knowing serial number, can't answer accurately, but if by 'reach cruise' you mean at Wide Open Throttle (WOT) then you have always had a serious propping issue. The method to pick a prop is by choosing pitch based on a target RPM at WOT. Your motor is most likely designed to run maximum RPM of 4800 or 5000 RPM. You need to prop it to reach the max at WOT when you are running 'light', meaning with just you and a minimum of gear. That way when running heavy, with a load people and gear, you can still reach a reasonable 4400 or so RPM at WOT. This also means that at cruise, you are throttled back to run at 2/3 power or so, which is more efficient.

        Yes, you could have a tach problem. But you definitely have either a serious motor problem if all you can get is 2900 RPM or a far too big prop. If all you can reach is 2900 you are lugging that motor and it will not last long.
        17 Foot '90 Bluewater
        Alpha 1 3.0 Merc - repowered in 2016
        now - 4.3L Vortec - Alpha 1 GenII
        Honda BF100L kicker
        Engine custom - Transom 0D65328 - Drive 0D648864
        If you ain't fishin, then yer working too much.

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        • #5
          3200 is all we got at WOT in the past. Bought boat used 10 years ago. Serial # is 0k002927. WOT range is 4200-4600 I believe. Prop has some #’s on it. 033004 14x19. Not sure what that means

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          • #6
            Originally posted by Medmpmpn View Post
            3200 is all we got at WOT in the past. Bought boat used 10 years ago. Serial # is 0k002927. WOT range is 4200-4600 I believe. Prop has some #’s on it. 033004 14x19. Not sure what that means
            Running the boat at WOT for 10 years and only getting 3200 RPM, the motor is worn out
            Compression test
            94 Formula 27PC Custom 509MPI MEFI3 , B3 XR
            95 Rinker 232 w/ 7.4L Carb 0F425011 B1 0F486471
            07 Seadoo GTX
            Merc Adults Only VP Adults Only
            G-Dad always said "First Liar doesn't stand a chance"

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            • #7
              Originally posted by alldodge View Post

              Running the boat at WOT for 10 years and only getting 3200 RPM, the motor is worn out
              Compression test
              This is truth. Running it that way would be like putting those 6 foot tall monster truck tires on your pickup without changing gear ratio. Be fine as long as you were only traveling steep downhill. A boat is always under 100% load.

              Run a compression check. Weigh your boat. Find out what is going on and fix it.
              17 Foot '90 Bluewater
              Alpha 1 3.0 Merc - repowered in 2016
              now - 4.3L Vortec - Alpha 1 GenII
              Honda BF100L kicker
              Engine custom - Transom 0D65328 - Drive 0D648864
              If you ain't fishin, then yer working too much.

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              • #8
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                Thanks everyone. Not what I was hoping to hear but I’ll call the mechanic.

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