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  1. #1
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    1998 225 efi starting knocking while cruising

    my 1998 mercury 225 efi developed a knock while cruising around half throttle the other day. pulled all the plugs and found one that looked like this. weighing rebuild vs new powerhead vs rebuilt powerhead. I am pretty solid with a wrench as i do it for a living but just not boats. anybody have an idea what a new power head will cost me? also what should i be looking for when i take this powerhead off as a source of failure im thinking an injector clogged but not 100% sure if what on the plug is aluminum or carbon

    i could not find the serial number on the motor but did find this part number on the power head 37855211

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    #2
    Here’s the photos
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    #3
    Sorry first pics where really small
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  4. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #4
    Did you perform a compression test or open that cylinder up to inspect?

    Based on that engine's age- it's difficult to justify reman or rebuild unless you're doing the work yourself.


    Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
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    #5
    Plan is to do compression test tonight or tommorow if I remember to bring home my guage from work. This is a friends motor but I plan to do the work I can which is pretty much everything besides the machine work. We are pulling the boat out of the water Thursday to check it so trying to get a list of what to check while we tear down

  6. Major Flagelator Gamblinman's Avatar
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    #6
    You need to determine exactly where that knock is coming from. Many lower unit sounds can transmit up and sound like they are coming from the powerhead.
    "Better to remain silent and be thought a fool than to speak out and remove all doubt."

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    #7
    If your motor needs major repairs, then I would buy a used Optimax!



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    #8
    Quote Originally Posted by Gamblinman View Post
    You need to determine exactly where that knock is coming from. Many lower unit sounds can transmit up and sound like they are coming from the powerhead.
    i agree need to figure out where the knock is i believe it is just the cylinder misfiring making the knock . going to compression test the motor tonight and look inside what i believe is cylinder 5( middle cylinder on left side of engine) with a bore scope and see what we have going on.

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    #9
    #5 is on bottom stb side side with starter
    .................................................. ...the scariest thing in life is the unknown ...................................

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    #10
    FECD5009-E0B2-4A65-A99F-94BA2B23392B.jpeg

    well borescope confirmed what I though had happened hole in the piston. Going to pull head tommorow and see how bad it is when we pull it out of the water

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    #11
    Also checked compression. In all 5 other cylinders for giggles and all are at 95-100 psi

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    #12
    948D8F53-4E7E-4C17-8880-6599A6A22A0B.jpeg
    well theres the end result

  13. Major Flagelator Gamblinman's Avatar
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    #13
    Time for a new powerhead. Need to determine what caused this initially.
    "Better to remain silent and be thought a fool than to speak out and remove all doubt."

  14. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #14
    Be interested in seeing a close-up of that #4 piston.


    Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
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  15. Member wmitch2's Avatar
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    #15
    What do the reeds look like ? You may get by with repairing that one cylinder and cleaning up the others. I'd definatly change the plugs and reeds and start running some Quick-Kleen with every fill up.
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    #16
    A 2 stroke is a lot easier to rebuild than a 4 stroke engine. Lot less parts. Eight or so years ago I rebuilt a 90 HP Yamaha 3 cylinder. Block bored, new pistons, new carb kits and water pump kit, new seals. I did reuse the crank, rod and wrist pin bearings. Cost $200 a hole or $600 total. My buddy now owns that boat and engine and still running flawless.

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    #17
    Quote Originally Posted by EuropeanAM View Post
    Be interested in seeing a close-up of that #4 piston.
    Here’s a close up I’m not sure if it whent lean or maybe cranked the piston. Head does not look damaged like a chunk was flying around70CD6331-5C71-40BB-826C-7D172399CF9E.jpeg

  18. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #18
    Tough to tell from that photo. Need to focus on the hole- and whether the edges appear to be broken/fractured or "melted"


    Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
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  19. Member lpugh's Avatar
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    #19
    Almost looks to me that the rod punched a hole from the bottem!
    Thank You Leon Pugh

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    #20
    You should have a full rebuild shop measure up that hole and be sure its not a re-sleeve job. Thats about 4-500 right there. Looks like at least .030 over to clean up that hole. Any deep grooves and you're... From the age of the motor I'd also be looking for 'out of round' holes that need to be bored. Like Don said, consider the age. I'd retrieve the hours from the ECM, plan on fiber reeds, flo test and clean injectors and fuel rail, Be sure to check your lower swivel pin on DS housing for slop chatter and play. If the splines are bad it's gonna get too expensive. (Tho I may have a cheap fix for that). - And check upper and lower motor mounts. Check your gear oil and consider how well it shifts etc.