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Loud knock and engine lock up on start up...

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  • Loud knock and engine lock up on start up...

    I have a 1991 Mercrusier V6 4.3lx i/o. I have always had run on issues when turning the engine off, even on 91 octane. I was advised to check the timing. Timing was at 0 degrees. I adjusted it to the factory setting of 8 degrees BTDC, also put in oem spark plugs gapped at .035. The idle went up pretty high when i advanced the timing, so i adjusted that to about 750 rpm out of water. I tightened the ditributor bolt and re-checked the timing for accuracy. Went to the lake and the boat ran significantly better, but was still idling high. I planned on adjusting it after a family swim. Run on was greatly reduced at shut down. 40 minutes later went to start the boat and the motor turned over maybe 3 times and then a load knock sound and lock up. No other sounds just stops cranking. No armature sounds from the starter. I did a complete visual of everything in and out of the boat, everything seems normal. Over the next hour i tried it 5 more times with the exact same result. Motor turns over about 3 times then the load knock and then it just stops cranking. I've owned this boat for 14 years and do all my own maintenance on it. All fluids are fresh and full, new water impeller, new battery. Anybody have any ideas of where I should start???


  • #2
    Re: Loud knock and engine lock up on start up...

    Pull the spark plugs out and see if it keeps cranking. If you still get the lockup, remove the drive and try it. If you still get the lockup, pull the engine and find out why.
    It sounds to me like you have a broken rod. Hope I'm wrong.
    Don S.

    sigpic

    Please, no PM's (Private Messages) regarding boat/engine problems.
    That is what the forums are for.
    Only forum/moderator issues will be answered in PM's.

    Comment



    • #3
      Re: Loud knock and engine lock up on start up...

      Welcome to Iboats.

      Sounds like you should start by pulling the plugs, and cranking the engine over to see if there is water in your cylinders.
      1976 Rogers Custom Boats 18' "Bonneville" W/ Chevy 454/ Berkeley 12JC

      Care Taker of 1976 Glastron V-184 Crestflight W/ Volvo 170B/280 I.O.
      & 1969 Starcraft W/ 1956 Evinrude 7.5hp Fleetwin.

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      • #4
        Re: Loud knock and engine lock up on start up...

        I vote for a failed head gasket. As the locked up water & air pressure drops down to almost nothing between crankings, it allows the piston to push out the water in a cylinder............That allows the engine to crank around till it is on the next compression stroke upward of that cylinder or cylinders.


        STOP CRANKING..........If I am right, you have NO oil on the piston or cylinder walls. Metal to metal scrubbing.

        Also..You may have water in the oil system & pan / pump.

        Comment



        • #5
          Re: Loud knock and engine lock up on start up...

          Head gaskets don't fail like that
          Don S.

          sigpic

          Please, no PM's (Private Messages) regarding boat/engine problems.
          That is what the forums are for.
          Only forum/moderator issues will be answered in PM's.

          Comment



          • #6
            Re: Loud knock and engine lock up on start up...

            Either broken rod or the timing is way to advanced, trying to push the piston down to early.

            Comment



            • #7
              Re: Loud knock and engine lock up on start up...

              I would remove all spark plugs and turn the engine over by hand and try to see if all pistons are moving a full range of stroke. If you have a broken rod that piston will not move.

              Comment



              • #8
                Re: Loud knock and engine lock up on start up...

                Pulled the plugs out of the boat today to find we brought more of Lake Pleasant home with us than we realized. Blown head gasket, cylinders 2, 4, and 6... We will probably do the work ourselves. Anything we might need to do, our watch out for would be appreciated... Thanks guys!!!

                Comment



                • #9
                  Re: Loud knock and engine lock up on start up...

                  Make sure you replace the starter bolts they tend to bend with 3 try's.

                  Comment



                  • #10
                    Re: Loud knock and engine lock up on start up...

                    Before you go pulling the head, do a leak down test. I'll bet there is no blown head gasket. When the engine dieseled, it sucked water back up the exhaust and into those cylinders.

                    Comment



                    • #11
                      Re: Loud knock and engine lock up on start up...

                      I got some good advice from my auto mechanic/fellow boater. He said I very much doubt that all 3 cylinders on the right bank would have water in them for a blown head gasket. He said the gasket would've had to fail in three places to have water in #2,4, and 6 cylinders. He said there's a gasket in between the exhaust manifold and the exhaust elbow that probably failed sending the water running down into the cylinders. If the exhaust vavles were opened when i turned it off, the water back flowed right into them. Then when i tried to start it later and it only would turn over like 3 times and lock. He called this a hydro-lock. So I'm going to take apart the exhuast elbow casting and inspect the gasket. Anybody have any thoughts on this new shed light???

                      Comment



                      • #12
                        Re: Loud knock and engine lock up on start up...

                        Don't use any water, and with the spark plugs out, will the engine crank over more than 3 times? If so, the put the spark plugs in, still no water and crank the engine. Does it keep cranking?

                        Go one step at a time and determine the cause and what you will do to fix.
                        Don S.

                        sigpic

                        Please, no PM's (Private Messages) regarding boat/engine problems.
                        That is what the forums are for.
                        Only forum/moderator issues will be answered in PM's.

                        Comment



                        • #13
                          Re: Loud knock and engine lock up on start up...

                          Hey Don S, I pulled the new plugs off and sprayed some flogging oil in the right bank cylinders, the ones that got wet. Pulled the plug wires off and cranked it over for a minute. Every seems to be working fine internally. no noise just cranking...

                          Comment



                          • #14
                            Re: Loud knock and engine lock up on start up...

                            Carefully remove the riser on that side and check the gasket. You should hope that it's bad cause that's a cheap repair. If it is bad get new gaskets with the graphite coating on them and torque em to 30 foot lbs.
                            If the gasket surfaces aren't the leak you might need to change out your risers and manifolds. While the risers are off do a careful check for leaks.
                            Also, get the mating surfaces nice and flat so the gaskets seal tightly. Don't buy those green gaskets, they don't seal well.
                            1989 Baja Sunsport 196, Mercruiser 5.7, Alpha 1
                            Moultonborough Neck, Lake Winnipesaukee, NH

                            Comment



                            • #15
                              Re: Loud knock and engine lock up on start up...

                              Already bought the gaskets for the exhaust elbows. The brand is Sierra, should I use some gasket sealer when I re-install? The right side elbow (problem side) didn't show a clear cut gasket failure, but it's definitely 20 years old and not in good shape. My manual says to soak the elbows in muartic acid for 1 1/2 hours to clean them up. So I'm working on getting it ready to re-install. Anybody ever check there exhaust shutters before???

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