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Mercruiser 7.4 MPI Overheated!-gctid826265

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  • Mercruiser 7.4 MPI Overheated!-gctid826265

    Well, we just had our first ever "breakdown" and had to be towed in. Thankfully I opted for the BOAT/US "Unlimited" Tow option... best $149 I ever spent. The problem started while we were cruising at about 1800 RPM/8 MPH. We heard a "rumbling" or "thrumming" sound from the engine, so we came to slow idle in gear. Quieter, but definitely something there. And then I noticed the temp gauge, instead of it normal just-below-175 degrees, was climbing steadily up to 185-190. We shut her down and checked the engine for obvious signs of trouble, but nothing visual. I am sure the overheat was real though, the risers were the hottest I've ever felt them. I thought about restarting it, but in its elevated temp state, I thought we might get vapor lock. So we called for help. We were only 5 miles from our marina, and the Vessel Assist got to us in 35 min.

    My mistake I think, was not letting her cool way down, and retrying. I say this because after they brought us into the marina, we started her up to park in our slip. I let her run for almost 20 min. after that, watching and feeling the temp while flushing with fresh water/Salt-Away. She stayed cool as a cucumber. I even advanced the throttle in neutral to see if higher RPMs would show signs of trouble. Nothing! BTW we just had the raw water pump replaced a few months ago when we pulled the motor to replace the steering pin/seal.

    So my theory is this. We picked up a piece of kelp, which somehow covered the outdrive intake holes. The holes would have kept suction on the kelp leaf until we shut it off- even in neutral. We have hit kelp before at higher speed (the Bravo 3 has a "notch" at its leading edge which can hold on to a kelp rope), but never has it blocked water intake- just caused a funny-looking wake. Since we were at much slower speed, I think it's possible the kelp wrapped around the leg. We will sea-trial it carefully just outside our marina before calling it okay.

    Anybody ever have something like this happen? Any ideas on what else it could have been?
    Jeff Prime
    "Optimystic Prime"
    2000 Bayliner 2855 Ciera Sunbridge
    Anacortes, WA

    "The reason you get lost in your own thoughts is because it's such unfamiliar territory"

  • #2
    Are you in salt water?

    How old are the exhaust manifolds and risers?
    Huntington Beach, California
    2018 Element 16
    Currently looking for 32xx in South Florida
    Former Bayliners: 3218, 2859, 2252, 1952


    • #3
      Same thing happened to us, no apparent reason, but the temp alarm sounded. We were in a shallow bay in San Pedro, CA, windier than hell, 20-30 mph. So the first thing I did was drop anchor, then opened the engine hatch. No signs of trouble, no steam, no seawater, no coolant. We let it cool for 30-40 minutes, then all was fine. We cruised slowly for about 30 minutes, then on plane for the last 30 minutes to our marina, no problems. We came to the conclusion of kelp or a plastic bag blocking the raw water inlets on the drive.
      Jeff & Tara (And Hobie too)
      Lake Havasu City, AZ
      Current: 2022 Sun Tracker Sport Fish 22 XP3 w/ Mercury 200
      2000 Bayliner 3388 Cummins 4bta 250s (SOLD 2020)
      2000 Bayliner 2858 MCM 7.4 MPI B3 (SOLD 2018)
      2007 Bayliner 305 MCM twin 350 Mag B3s (SOLD 2012)
      2008 Bayliner 289 MCM 350 Mag Sea Core B3 (SOLD 2009)
      And 13 others...
      In memory of Shadow (7-2-10,) and Ginger (5-11-21.)
      Best boat dogs ever! Rest in peace girls...


      • #4
        Up here we have areas of eel grass that will stuff up the intake. I was talking to a captain of a vessel assist boat and he said it happens to him as well, often behind (west side) Bainbridge island, and he usually throws it in reverse and raises/lowers the outdrive to clear it..
        Johnson Point, Olympia, WA
        1989 2855
        Horizon 6.2 and Bravo II


        • #5
          Yes grass or trash can block the inlet holes on the drive. The grass in my area can build up going through shallow areas that the boat will not plane. I have to stop it and gun her in reverse then all is good. This leaves a large amount of grass floating in front of the boat so I steer to the side to avoid it.
          1997 Silverton 362, 7.4 Crusaders
          1997 Maxum 2400 SCR, 5.7LX Bravo II



          • #6
            Yes, we are in salt water. Exhaust manifolds probably original, and risers two years old. I do need to replace the down pipes (the ones going into the outdrive) as they showed some wear upon engine disassembly. I have them in a box ready but have yet to pull down the boots and put the new ones in.
            Jeff Prime
            "Optimystic Prime"
            2000 Bayliner 2855 Ciera Sunbridge
            Anacortes, WA

            "The reason you get lost in your own thoughts is because it's such unfamiliar territory"


            • #7
              This happens to me alot when i leave my river. The leg will pick up some grass. Put in reverse, look for floating grass. If u have to shut down, raise drive and check it.

              Also as far as noise, that happened once too. I had a giant gob of seaweed from the detroit river. I couldnt even get on plane and powered down right away and checked it.

              Very use to this now. I ALWAYS check the drive for blockages before hitting the throttle
              1993 formula pc 31 twin 454 bravo 2
              1989 2655 cierra 5.7 omc cobra
              2014 "searay" tandom trailer

              Anchor bay clinton river


              • #8
                We get barnacles blocking the water intakes here on moored boats sometimes you have to split the drive to clean em out.
                88 Four Winns 200 Horizon 4.3 OMC
                98 Jeep Grand Cherokee 4.0/Selectrac
                07 Jeep Grand Cherokee 5.7 Hemi/Quadradrive II

                Long Island Sound Region