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(39)98 Merc 350 Mag MPI Bravo 3 Low on power, Stalls...-gctid816627

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    (39)98 Merc 350 Mag MPI Bravo 3 Low on power, Stalls...-gctid816627

    Boat is new to me.

    When I first launched it, it leapt up on plane and was fast.

    Sat in a slip for a few weeks getting some things worked on and sorted out. New fuel, and the only engine work was replacing the alternator. Took her out for a cruise, and she was noticeably way low on power. Barely got out of the water and would not get over about 2700 rpm....other than that ran smooth. Took it back to the slip.

    A week later took her out again, power was better but still low. Slowly got to about 3700 rpm at wot...better but still way low. Then after cruising for about an hour and a half, she suddenly died. Went to neutral and she had trouble restarting but did restart with the gauges flopping all over the place, running rough, then smoothing out and running like it did before.

    Took her out today, noticed low on power, 2700 rpm at wot, then suddenly climbed to 3200 rpm. Watching the gauges closely, I noticed they would all slightly Flicker every few seconds. Then again she died. Started up again like before but rough with the gauges all over the place before smoothing out...tried to add more power and the guages all went a little nuts, and the engine backfired slightly and tried to stall, but kept running when I reduced power to about 1100 rpm and cruised at that speed back to the dock, watching the guages just slightly flicker ever 30 seconds or so, not consistent, all the way in. At the dock, under no load, she runs perfect. No detectable issue at all. Only under load does she act up.

    I am hoping it's something simple like a distributor cap gone bad sine it ran great then didn't....the boat had been in storage for a few years before I picked her up.



    The change from when you bought it was the alternator. With the gauges acting the way they are, I am immediately suspecting that.
    Matt Train
    BOC Site Team
    Chicagoland, IL


      it never occurred to me that an alternator could cause these symptoms.

      If I disconnect all the wires from the alternator and run it as a test, will I hurt the alternator? What would you suggest as the best way to test it?



        did a little research...but help me out.

        It seems Alternators would cause rough running but typically if the batteries are shot or the voltage is low.

        I have all new batteries and the voltage read is great, no low voltage issues at all.

        Would there be other possibilities in the alternator that could cause this?

        I am going to change the distributor cap, and if that doesnt fix it, I will disconnect the alternator and run her hard...

        Any other suggestions?



          These MPI engines are very "clever". Plug the scan tool in and see what codes you get
          1999 Bayliner 1750 Capri. 3l Mercruiser Alpha

          2014 Yamaha VX Cruiser



            I think I'm gonna need the smart guys here...

            It's throwing codes 42 and 43, which is the knock sensor and ignition module. I have both parts on order. Now for the curve ball...

            Download_Complete's comment about the alternator stuck with me, so I disconnected the alternator and all the guage flickering, and stalling symptoms vanished. It's still low on power, which I believe is a bad knock sensor retarding the timing too much (the code is that it is sensing a continuous knock).

            So now that the brand new alternator is identified as the culprit of the stalling problem, what now? Is it as simple as a bad (brand new) alternator or could it be something else in the system?



              Forgot to add...I changed the cap and rotor too. They were bad. Likley original from '98. Once I changed those, the stalling problem was worse and I could get it to do it at the dock. Guages jumping then stalling...

              Then I tested it with the disconnected alternator....


                As I am thinking about this and studying wiring diagrams, could this be an ignition switch problem? The previous owners were NOT good at wiring, and the ignition switch is not original and the kill switch has been bypassed. Thoughts?


                  In case anyone is wondering, I swapped in a new marine ignition switch, as the PO had a plastic automotive cheapie installed. Reconnected the alternator and poof! Stalling problem solved!!!!

                  Now I have a ignition module being delivered and I don't need it. I hope I can get a refund.

                  Anyway, just in case anyone was's fixed!


                    Just seeing this thread now, wish you had more reply's ... not typical, everyone is out boating living the summer life we waited for all winter.

                    Don't give up on us, your post will help someone down the road.

                    Thank you for the update !
                    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