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7.4 Mercruiser: Surging Engine. HELP!!!!!

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    7.4 Mercruiser: Surging Engine. HELP!!!!!

    Hi All,

    I was using the boat for 3 months without any issues. Then I was out cruising along approx. (24 MPH @ 3400 RPM) and the engine seemed to start surging when I went to WOT as if I was losing a cylinder or two. I checked the engine and as I was looking it over, checking the wires, distributor cap etc. while it was running and I just barely touched the top of the Cap and the engine died. I opened the cap and checked the Rotor. I found the little slot in the inside of the Rotor which fits into the top of the distributor, was sheared off and the Rotor was just spinning freely. We got towed in.

    I ran around the corner and bought a new Sierra Cap & Rotor kit. I replaced the Rotor since I replaced the Cap last Fall. I went out to TEST RUN the boat and it seemed fine, went to WOT and back to cruise, after about 2-3 mins. around 3400/3600 RPM it started to surge again. I stopped running the boat and sat in the water for about a few minutes. Then I took off and opened her up and again it ran OK. While running now WOT for approx. 45 seconds and bringing it back to cruise approx. 3400RPM now after 3-5 mins. of running fine, it starts to surge again. Back to the dock again I go.

    I replaced the Cap now that came with the New Cap & Rotor Kit and back out I went. Seemed to run OK for about 5-6 mins. then it started to surge again. Back to the dock again. While it was running I tried to wiggle it and see if there was any play and I heard a click. It was the Rotor hitting the inside of the Cap again.

    I had my mechanic come take a look at it. We found that one of the screws that locks down one side of the cap was slightly loose. We turned the screw and although it’s tight it never stops turning. We found that while pushing down on the screw hard while turning it got it to really bite hard and now it was very tight.

    Out again I go to TEST Run it and long & short of it is, it is still surging after a few mins. of running. Again it seems to run OK for a few 4-6 mins. and then it’s surging again. It may run OK and then surge and then run OK several times every time I TEST Run her. The issue seems to be intermittent as far as the times it runs before it surges and if I go to WOT as well.

    My Mechanic said I should try replacing the Cap & Rotor again since the previous time out, I must have shave a little bit off the inside of the cap when I tried to wiggle it a bit to see if the cap was loose and it was. But he says I should try replacing the Cap & Rotor again since when I tried to wiggle the Cap the Rotor tapped the inside of the Cap. We saw evidence of some very fine curly, hair-like shavings before we realized there was a slightly loose screw. Since we know that now, he says to get a New Cap & Rotor and make sure the screws are tight and try it again.

    We did inspect the Distributor for any sloppy play and it seemed very tight with NO Play in it. Unless the play happens while its only running? The one screw hole may be slightly stripped as these plates the screws go into are extremely thin.

    Also just another Note: There seems to be a slight surging effect while Idling down the canal. I can also see the Tach at 800 RPM and it bounces up to 900 RPM and back down to 800RPM and I can hear it pick up and drop slightly as well. Again similar to a Cylinder cutting in and out.

    Does anyone have any suggestions or ideas of what may be causing this issue? Since the first day we got stuck and I found the slot was broken off the inside of the Rotor that fits into the slot in the top of the Distributor we feel that we are on the right track and do not think it’s the fuel injection related etc. Stumped on this one......

    One more Note: The Plugs & Wires were replaced the year before last.

    Thanks & Look forward to your responses.


    Some rotors got to be glued down.
    Be good, be happy, for tomorrow is promised to no man !

    1994 2452, 5.0l, Alpha gen. 2 drive. Sold ! Sold ! Sold !

    '86 / 19' Citation cuddy, Merc. 3.0L / 140 hp 86' , stringer drive. Sold ! Sold ! Sold !

    Manalapan N.J


      if its not actually misfiring, im going to say you have a fuel delivery issue... probably a plugged filter which is not allowing enough fuel thru to keep the engine running steady at WOT....

      Salem, OR

      1989 Bayliner 2556 Convertible
      5.7 OMC Cobra - 15.5x11 prop
      N2K equipped throughout..
      2014 Ram 3500 crew cab, 6.7 Cummins
      2007 M-3705 SLC weekend warrior, 5th wheel
      '04 Polaris Sportsman 700 -- '05 Polaris Sportsman 500 HO
      Heavy Equipment Repair and Specialty Welding


        What year is the engine? Fuel pump mounted on the raw water pump? If so has the oil chamber been maintained? If not the fuel pump arm may be warn preventing proper fuel pressure.
        1997 Maxum 2400 SCR 5.7LX Bravo II



          Originally posted by Nauti_Mike View Post
          What year is the engine? Fuel pump mounted on the raw water pump? If so has the oil chamber been maintained? If not the fuel pump arm may be warn preventing proper fuel pressure.

          Hi Guys,

          The Boat is a yr. 2000 Ciera Sunbridge 285.. 7.4 Mercruiser.

          As far as the fuel - I replaced everything that should be done in the Spring and then in the Fall. I am usually very meticulous about the proper care. I have it in a Marina and have them work on it as well as having a private Mechanic. I always have it maintained properly. But this one has us scratching our heads.

          I always replace the Oil and filter in the fall and the Fuel filter in the Spring, along with all of the proper winterizing / fogging of the engine in the Fall and getting it ready and cleaned up in the Spring..

          The Plugs and wires were replaced 2 years now I am on the 3rd Cap & Rotor. Now going for the 4th dues to it getting scraped while out last time.

          I am replacing the Cap & Rotor and adding a new screw with a Lock nut and washers to the underside of the Distributor plate on the screw that appears to be slightly stripped , (just for safety)..

          Thanks for the responses so far guys.

          I will post how I make out... (If the rains ever stops) LOL...



          • Nauti_Mike
            Nauti_Mike commented
            Editing a comment
            That sounds great but you did not answer my question, do you have a fuel pump mounted on the raw water pump? These are know to be troublesome.

          Does sound like a possible cap/rotor issue. I’d also replace the plugs just cause that’s cheap and easy. May find s plug doesn’t look right.
          Edmonds, WA
          "THE FIX"
          '93 2556 5.7 Bravo II 2.0:1 18 1/4x19 P
          (.030 over-Vortec top end-part closed cooled)
          The rebuild of my 2556
          Misc. projects thread