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  1. #1
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    Default 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. #2
    Honorary Moderator Emeritus Don S's Avatar
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    Default 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.


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  3. #3
    Commander RogersJetboat454's Avatar
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    Default 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.

  4. #4
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    Default 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.

  5. #5
    Honorary Moderator Emeritus Don S's Avatar
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    Default Re: Loud knock and engine lock up on start up...

    Head gaskets don't fail like that
    Don S.


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  6. #6
    Chief Petty Officer Idlespeedonly's Avatar
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    Default 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.

  7. #7
    Petty Officer 1st Class T_Herrod's Avatar
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    Default 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.

  8. #8
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    Default 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!!!

  9. #9
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    Default 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.

  10. #10
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    Default 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.

  11. #11
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    Default 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???

  12. #12
    Honorary Moderator Emeritus Don S's Avatar
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    Default 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.


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  13. #13
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    Default 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...

  14. #14
    Commander NHGuy's Avatar
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    Default 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
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  15. #15
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    Default 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???

  16. #16
    Rear Admiral Scott Danforth's Avatar
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    Default Re: Loud knock and engine lock up on start up...

    DO NOT USE THE SIERRA GASKETS they are called "green leakers" for a reason. buy OEM grey composite gaskets. they are only a bit more, however wont leak.

    post a picture of the gasket mating surface of boat the manifold and elbows.
    1988 Cruisers Rogue 2420 - VP AQ271C / 290DP "Rock'n Along"
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  17. #17
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    Default Re: Loud knock and engine lock up on start up...

    The mating surface of both elbow and manifold have been cleaned with a nuematic wire brush. So we are clean as a whistle. My Seloc repair manual says to put some gasket sealer on both sides of the gasket. Any thoughts???

  18. #18
    Lieutenant Commander superbenk's Avatar
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    Default Re: Loud knock and engine lock up on start up...

    Pretty sure I remember my manifold gasket kit included both regular paper & graphite-infused gaskets & included instructions to only use gasket sealer on the non-graphite-infused (light gray not dark gray) gaskets. The main intake/exhaust gasket that went up against the head and the one between the manifold and the carb were the graphite-infused ones and I didn't use sealer on them. The end caps I put sealer on. I can't remember if the riser gasket had sealer or not (I don't think it did now that I think about it).

  19. #19
    Moderator Bondo's Avatar
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    Default Re: Loud knock and engine lock up on start up...

    Quote Originally Posted by Buck Naked View Post
    The mating surface of both elbow and manifold have been cleaned with a nuematic wire brush. So we are clean as a whistle. My Seloc repair manual says to put some gasket sealer on both sides of the gasket. Any thoughts???
    Ayuh,.... Now, take a Flat File to 'em, to see the Highs, 'n Lows.... They Gotta be Dead Flat...

    'n,... As noted, ditch the green leakin' gaskets, 'n buy a set of OEM, Mercruiser metal/ graphite gaskets, which go on Dry...
    Any Grease is Better,..... Than No Grease at All.......

  20. #20
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    Default Re: Loud knock and engine lock up on start up...

    I just wanted to chime in on this one. I had the same thing happen to me last weekend. I was on the water and took my kid for a kneeboard ride. When i shut down the engine after his ride the engine did like a chuga chuga run on before dying. after that the motor would not turn over because it sucked up water from the exhaust into the cylinders and the water cannot be compressed during the compression stroke therefore it appears to be seized but actually it is not. nor are there any blown gaskets. I pulled the plugs and poured oil in and cranked the contamination out. I repeated until only clean oil was squirting out when cranking with no plugs. I put in new plugs and whammo! all good again. I would recommend trying that before barreling ahead changing gaskets unneccessarily. You will be opening a can of worms on a boat that old.

    Cheers!

  21. #21
    Lieutenant Junior Grade ktbarrentine's Avatar
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    Default Re: Loud knock and engine lock up on start up...

    Quote Originally Posted by Buck Naked View Post
    The mating surface of both elbow and manifold have been cleaned with a nuematic wire brush. So we are clean as a whistle. My Seloc repair manual says to put some gasket sealer on both sides of the gasket. Any thoughts???
    My thoughts?....get rid of that worthless Seloc repair manual!!!

  22. #22
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    Default Re: Loud knock and engine lock up on start up...

    toolm8kr,
    I just am at a loss why I only had water sucked back into the right bank of cylinders. Why did it not suck it back to the other side? did you have water in all your cylinders???

  23. #23
    Commander RogersJetboat454's Avatar
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    Default Re: Loud knock and engine lock up on start up...

    Quote Originally Posted by Buck Naked View Post
    toolm8kr,
    I just am at a loss why I only had water sucked back into the right bank of cylinders. Why did it not suck it back to the other side? did you have water in all your cylinders???
    I don't know if I missed this in previous posts, but did you check the condition of the flapper valve in the exhaust?
    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.

  24. #24
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    Default Re: Loud knock and engine lock up on start up...

    Quote Originally Posted by ktbarrentine View Post
    My thoughts?....get rid of that worthless Seloc repair manual!!!
    What would you suggest? I would think Mercruiser would sell one, wonder how much???

  25. #25
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    Default Re: Loud knock and engine lock up on start up...

    Quote Originally Posted by toolm8kr View Post
    I just wanted to chime in on this one. I had the same thing happen to me last weekend. I was on the water and took my kid for a kneeboard ride. When i shut down the engine after his ride the engine did like a chuga chuga run on before dying. after that the motor would not turn over because it sucked up water from the exhaust into the cylinders and the water cannot be compressed during the compression stroke therefore it appears to be seized but actually it is not. nor are there any blown gaskets. I pulled the plugs and poured oil in and cranked the contamination out. I repeated until only clean oil was squirting out when cranking with no plugs. I put in new plugs and whammo! all good again. I would recommend trying that before barreling ahead changing gaskets unneccessarily. You will be opening a can of worms on a boat that old.

    Cheers!
    Yeah it turned into a cluster. But i feel good that all the rust and junk is outta there. I compression tested all the cyliders, ranged from 150 to 165psi. Fresh oil and filter and she fired right up. With the timing adjusted from 0 to 8 BTDC and the idle adjusted down to 750rpm the run-on problem seems to be a thing of the pass. I still want to inspect the exhaust shutters...

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