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Thread: Mercury 75 hp Optimax Sounding Alarm

  1. 02-27-2012, 12:09 PM #1
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    Default Mercury 75 hp Optimax Sounding Alarm


    On Saturday I ran my boat up in some shallow water (2ft and less). The bottom was a heavy sandy, silty muck. I got hung up...and to get out I resorted to plowin through the muck. the prop was definately just diggin in even though I trimmed up as high as I could. I hate when I do stupid stuff.

    I got out but I went back out in the main channel...and started to get back up to speed, the Alarm started a continuous alarm. At that point I shut everything off....trimmed up out of water...inspected...it looked ok...

    But the rest of the day....It would not let me get to 2000RPMs with out alarming....took me an 1hr 40min to get back to launch. Sometimes I would be putzin along about 3-4 mph and the alarm would go off. I would shutter down....restart it put it in gear....it would sometimes alarm 3 or 4 times then finally stop and I could putz along....
    Motor runs perfectly, Water pump is spitting water as it should...I had just topped of the oil resevoir...but popped the top and chacked it (just in case I dreamed I filled it!)
    I did notice the Tach would jump some.....

    Any way it was a long ride back to the launch.

    Do you think I spun the prop? Never had that happen....I haven't had a chance to look at it yet. Any ideas? I am hopin like crazy I didn't tear up anything major...
    Thoughts?
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  2. 02-27-2012, 12:16 PM #2
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    Probably nothing major. Maybe just silt in the water passages or debris in the impeller either reducing its efficiency or it has worn it to where it can't pump well enough to keep up at higher speeds.

    If your impeller hasn't been changed in a couple of years, you might as well just replace it since it needs it and swapping it out will almost certainly cure your problem. If the impeller is new, you might try using the flush port on your outboard to flush the cooling system with a hose.

    The tach jumping could be a spun prop. That's not catastrophic either. You can check for that by using a Sharpie to make a mark from the hub to the prop, running the boat for a while and then comparing the alignment of the marks.
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  3. 02-27-2012, 12:27 PM #3
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    Is replacing the impeller something a reasonably handy fellow can do himself?
    Also on the tach jumping....I would have expected to hear the engine rev when it was doin that...but it didn't...
    Thanks for the quick reply....I feel better...
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  4. 02-27-2012, 01:00 PM #4
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    Steady beep is overheating alarm. You could have damaged the impeller and/or sucked up some trash. Pull the thermostats to check for trash and back flush while you have them out.
    Fair Winds and Following Seas

    Bill H. PTC USN Ret
    Chesapeake, Va

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  5. 02-27-2012, 01:05 PM #5
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    Quote Originally Posted by Crappieawlogist View Post
    Is replacing the impeller something a reasonably handy fellow can do himself?
    Also on the tach jumping....I would have expected to hear the engine rev when it was doin that...but it didn't...
    Thanks for the quick reply....I feel better...
    Search on youtube for Mercury outboard impeller or water pump and you will find several good demos. My only additional comment is to ignore what some of them say about using grease on the impeller. Petroleum grease can damage rubber. Use a silicon based grease designed for O-rings instead. It might not make a difference, but taking care of a boat is hard enough without making it worse yourself.
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