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  1. #1
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    Merc 250 efi only firing on 5 cylinders

    looking for a little help on one of my 1998 merc 250 efiís. I have a pair of them on the back of my boat one runs great the other is running a bit rough.

    Went to do some yearly maintenance, pulled the plugs(champion ql77cc) and found that number 6 cylinder, bottom left was all gummed up and had fresh fuel on it. Turns out itís not firing after a spark test, Iím getting only 5. The bottom left is the one not firing.

    I got a new coil, checked all the grounds, ohmíEd out the crank position sensor, ohmíed all the rest of the coilís, omhíed out the stator bobbins, checked continuity on the grounds, stater wire, trigger wire, stop wire, swapped the ecm ignition module with the other motor and nothing. Did a compression test and all came it at 115psi.

    This one has me stumped, any suggestions? I went through the service manual and performed all the tests under 2A ignition troubleshooting and havenít been able to find the culprit.

    Any suggestions or help would be much appreciated!

  2. #2
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    Gummed up how? black and oily, milkshake goo or carboned up?
    "One of the penalties for refusing to participate in politics is that you end up being governed by your inferiors". Plato .

  3. #3
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    Very black and oil with fresh fuel pretty much dripping off of it. The other five look very consistent and uniform with just carbonation.

  4. #4
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    Does it have spark on that cylinder?,if yes check the reeds...

  5. #5
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    Quote Originally Posted by Chris Carson's Marine View Post
    Does it have spark on that cylinder?,if yes check the reeds...
    I performed a spark that and found that cylinder to have no spark at all...

  6. #6
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    Check the connector pins for that cylinder all the way back into the computer. Just had a 225 efi with an intermittent spark and it was a pin connection problem.

  7. #7
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    Ok, will take a look and look into the connection pins/connectors.

  8. #8
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    So i took a look at the pins and they all "seem" to be making good contact. Also checked continuity back to the ECM on all the connecting pins.

    Something very strange, I hook all the plug wires back up to quicksilver spark test board and as I cranked the engine, I started unplugging the other cylinders off the quicksilver spark tester and all of a sudden #6 started firing. However with all six hooked up it will not fire at the same time. Seems like something is shorting out along with way or it's not getting enough power to fire all the CDM's at once?

    Any suggestions?

  9. #9
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    they switch between cdms on these motors...no fire on one may mean the cdm it switches with is bad ...pull all the primarys ...hook each back up and test for spark one at the time...see which one doesn't work

  10. #10
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    You need to check each cdm individually. A faulty ground on one can drop spark on another.

    One of them is bad

  11. #11
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    Quote Originally Posted by RSWORDS View Post
    You need to check each cdm individually. A faulty ground on one can drop spark on another.

    One of them is bad
    Mercury decided to drive us nuts with those..............
    "One of the penalties for refusing to participate in politics is that you end up being governed by your inferiors". Plato .

  12. #12
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    After testing the spark on the CDM's individually one by one on my spark tester and cranking the engine over, it looks as if I am getting spark on that cylinder.

    After doing a little more testing, I decided to check the volts on me throttle position sensor and I believe I am getting abnormal results. Here are my finding's, I used a digital multi-meter with two wires plugged into the connector, black is on the ground black wire and red is on the blue wire with a red stripe, does this sounds correct?

    I am getting 3.80 volts at idle and 2.13 volts at WOT.


    I have read in the manual I should be getting .095 +/- .05 at idle and 3.80 at WOT?

    Any suggestions? Does anyone think the sensor is shot?
    Last edited by Dklongbomb; 07-10-2019 at 09:55 PM.

  13. #13
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    Quote Originally Posted by Dklongbomb View Post
    After testing the spark on the CDM's individually one by one on my spark tester and cranking the engine over, it looks as if I am getting spark on that cylinder.

    After doing a little more testing, I decided to check the volts on me throttle position sensor and I believe I am getting abnormal results. Here are my finding's, I used a digital multi-meter with two wires plugged into the connector, black is on the ground black wire and red is on the blue wire with a red stripe, does this sounds correct?

    I am getting 3.80 volts at idle and 2.13 volts at WOT.


    I have read in the manual I should be getting .095 +/- .05 at idle and 3.80 at WOT?

    Any suggestions? Does anyone think the sensor is shot?
    A tps would not make one cylinder not fire. Best to address one thing at a time.
    "One of the penalties for refusing to participate in politics is that you end up being governed by your inferiors". Plato .

  14. #14
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    I have figured out the spark issue and is now firing on all 6 cylinders.

    The engine is definitely running more smooth at idle. However, if I rev the engine up a couple times and pull back the throttle the engine hesitates and acts like it’s going to die before regaining the idle speed again. Any suggestions? help would much be appreciated.

  15. #15
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    Howabout you share the spark solution?

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