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Mag 350 not revving to max, rough running

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    Mag 350 not revving to max, rough running

    Hi everyone, my 2008 merc started acting up last summer. It was preceded by a warning horn sounding at startup and complaining low gear oil level. Oil added. Issue with a leaking drive oil reservoir. Drive ok. After this particular incident the engine did go only to 3k rpm with hesitation. I though engine went to limp mode or I got water in the fuel since topped off recently. Both ruled out later.
    After a few days engine was almost as good as it used to be, but still I was not happy, running just a tad rough. Spark plug wires checked and merc guy checked with computer, no findings. Boat went to plane ok but still not the way it used to. Drive problem suspected. The upper part of the drive replaced with lots of cash, some blue coloring near the bearing.. Running almost ok, but..
    Took the boat out for a vacation trip and on the way home engine gave only 3k rpm, smelled gas in the exhaust and could not plane, limped to home port and computer plugged in, no findings. Btw, idle was just fine all the time. After a week mechs changed the sparkplugs - bingo! Previous plugs were only 2 yrs old, 80 hrs on them, but two plugs were judged bad. So my advice is change your plugs early and save a lot of headache and cash. Those computers do NOT show ALL ignition or fuel problems and basic mechanics judgement and experience is still of good value. Have a great summer!

    #2
    How were the 2 plugs judged bad, by look? There may be a reason they went bad like the injectors are leaking or an exhaust leak allowing water into those cylinders.
    1997 Maxum 2400 SCR 5.7LX Bravo II

    Mike

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      #3
      Hi Nauti_Mike,
      the mechanic said by look, sooth and looking wet, no talks of water entering the system though. I thought that maybe the engine went to limp mode after the initial warning of low oil level in the drive oil reservoir. How is the limp mode achieved? Cutting down power off the engine, by what? Maybe affecting ignition and fouling the plugs? After changing the plugs the engine has run about 10 h perfectly. Good questions Nauti Mike. I am only speculating.

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        #4
        Originally posted by Kivikari View Post
        Hi Nauti_Mike,
        the mechanic said by look, sooth and looking wet, no talks of water entering the system though. I thought that maybe the engine went to limp mode after the initial warning of low oil level in the drive oil reservoir. How is the limp mode achieved? Cutting down power off the engine, by what? Maybe affecting ignition and fouling the plugs? After changing the plugs the engine has run about 10 h perfectly. Good questions Nauti Mike. I am only speculating.
        My understanding is low gear lube doesn’t result in going into limp mode however things like the knock sensor detecting preignition does. You would need to look at the Mercruiser manual for the complete list. When in limp mode power is reduced to a certain rpm by affecting the injector timing and ignition advance. Guess I would just keep an eye on how she runs over the next 50+ hours.
        1997 Maxum 2400 SCR 5.7LX Bravo II

        Mike

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