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  1. #1
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    Mercury 3 Liter issues (after rebuild)

    Hello,

    I recently had a Mercury 250 EFI (from 1997) rebuilt. Although it runs, I'm having several issues with it:

    1. When on a plane in smooth water, sometimes the rpms will go down by 400 fairly quickly, then seconds later go back up, and continue this trend (without throttle being touched)

    2. Below 1,550 rpm, the engine vibrates the boat violently (same effect on your ears as driving on the freeway with one window down). As soon as you hit 1,550 rpm, it goes away

    3. It has horrible prop chatter when idling in gear

    4. Even though it's an EFI motor, it takes a couple turns of the key to get it running, and the motor usually sneezes at first.

    5. Just recently, the engine started making a loud clattering noise when not on a plane.


    As a little background, I've tested and disconnected the alternator, and it's not linked to any problems. I also tested all 6 CDM modules, and one of the module's coil secondary impedance bounces all over the place on my ohmmeter. Two of them also have bad corrosion where the spark plug wire is screwed in. Could this be causing some of these issues?

    Though I haven't done anything with the LU, I suspect there may be something wrong there as well.

    Any advice would be great!

  2. #2
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    Check for a intermittent miss.
    '06 Tracker All-Fish/Tournament V-18 90 Optimax, 46.8 gps Goal one complete reach 45 mph.

  3. #3
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    Quote Originally Posted by rdoaner View Post
    Hello,

    I recently had a Mercury 250 EFI (from 1997) rebuilt. Although it runs, I'm having several issues with it:

    1. When on a plane in smooth water, sometimes the rpms will go down by 400 fairly quickly, then seconds later go back up, and continue this trend (without throttle being touched)

    2. Below 1,550 rpm, the engine vibrates the boat violently (same effect on your ears as driving on the freeway with one window down). As soon as you hit 1,550 rpm, it goes away

    3. It has horrible prop chatter when idling in gear

    4. Even though it's an EFI motor, it takes a couple turns of the key to get it running, and the motor usually sneezes at first.

    5. Just recently, the engine started making a loud clattering noise when not on a plane.


    As a little background, I've tested and disconnected the alternator, and it's not linked to any problems. I also tested all 6 CDM modules, and one of the module's coil secondary impedance bounces all over the place on my ohmmeter. Two of them also have bad corrosion where the spark plug wire is screwed in. Could this be causing some of these issues?

    Though I haven't done anything with the LU, I suspect there may be something wrong there as well.

    Any advice would be great!
    Ohm the stator

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  5. #4
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    Corrosion is not good for any electronics, that will have to be attended to. If the plug wires for the affected coils are not new, best to get a new set of wires. As far as the funny ohm readings, You probably have a bad coill.

    Have you done a spark check with a spark checker tool set at 1/2"?
    "One of the penalties for refusing to participate in politics is that you end up being governed by your inferiors". Plato .

  6. #5
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    Quote Originally Posted by flabum1017 View Post
    Corrosion is not good for any electronics, that will have to be attended to. If the plug wires for the affected coils are not new, best to get a new set of wires. As far as the funny ohm readings, You probably have a bad coill.

    Have you done a spark check with a spark checker tool set at 1/2"?
    Thanks for the reply! I'm thinking of replacing that coil. Do you know if CDI electronics is any good? Trouble is OEM is so incredibly expensive.

    As far as a spark check, I have not. I don't even know what a spark checker tool is tbh

  7. #6
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    Outboard spark checker:

    Click image for larger version. 

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    Used to check strength of spark

    CDI is a good brand.

    Also, check the Stator (common problem) as Cali kid said.


    Bottom line, you have a misfire.... intermittent at high speed and seemingly constant at low speed.

    Once the ignition system is straightened out and if there is still a miss at low speed, you will be checking the reeds.
    "One of the penalties for refusing to participate in politics is that you end up being governed by your inferiors". Plato .

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  9. #7
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    this manual should be very similar to your engine
    http://www.hedges-uk.com/boat/Merc/o...nd%20Above.pdf

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  11. #8
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    Is there any reason I would have to remove the stator to ohm it?

  12. #9
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    Quote Originally Posted by rdoaner View Post
    Is there any reason I would have to remove the stator to ohm it?
    You do not need to remove it to test it.
    "One of the penalties for refusing to participate in politics is that you end up being governed by your inferiors". Plato .

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  14. #10
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    So I ohmed the stator, 5 of the 6 bobbins are just over 1,000 ohms, and one of them is at 800. Do you think that small difference really matters?

  15. #11
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    Quote Originally Posted by rdoaner View Post
    So I ohmed the stator, 5 of the 6 bobbins are just over 1,000 ohms, and one of them is at 800. Do you think that small difference really matters?
    Another thing, I replaced all of the CDM's with new ones, and the symptoms haven't changed

  16. #12
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    The stator needs replacing you will see that 800 change abit when it gets warm.

  17. #13
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    Quote Originally Posted by R.grover View Post
    The stator needs replacing you will see that 800 change abit when it gets warm.
    I'll be darned, I would've never guessed that would've mattered. Would you recommend OEM, or is CDI electronics more reliable?

  18. #14
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    Not sure if you can get oem still.

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