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  1. #1
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    Really rough idle 2004/2006 250XS

    Hi,
    This is my first time posting on this forum so I hope I have my protocols correct. I recently purchased a 2000 Donzi 35ZFC with two Mercury 250XS Optimax engines. The engines are a bit of a mix and match where the mid section and bolts ons were originally from a 2005 JP XL and the powerhead and mapping are from a 2006 250 XS 1E01045. It has taken me a bit of time just to sort that out. The previous owner bought them as a re-power from an offshore performance store in Fort Myers.

    One of the engines seems to run very well. Smooth and strong all the way through. The other seems to pull well but misfires and runs quite rough at idle. I should mention the ECU's are only indicating that the pair have 85 hours of running time. So far I have swapped coils around with no improvement, changed spark plugs with the NGK iridiums properly gapped. took out the add on Racor fuel water separators and checked compression which is even at 120 across all cylinders on both powerheads. My pressures are as follows:

    Air Fuel
    Running 94 108
    5 sec off 92 98
    10 sec off 88 88
    15 sec off 88 82

    I've read in the forums that the tracker valve is often to blame for poor idling issues. Does that sound like it might be the issue here? If not where would you start?

  2. Member
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    #2
    check for fuel vpors in air side of system , and reed inspection
    .................................................. ...the scariest thing in life is the unknown ...................................

  3. Member
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    #3
    Also worthwhile to have air and fuel injectors checked / flowed. Opti air injectors can be quite finicky and corrosion-prone.

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    #4
    I did a reed inspection and I can’t find any issues with the reeds. I also decided to compare the compressors against each other on the two outboards. When I spin the compressor on the engine with the poor idle it feels fine and has little play if any in it. However I can spin it (clockwise) and it doesn’t seem to build pressure after several rotations. When I spin the compressor on the other engine it builds pressure in a couple rotations to the point it gets hard to turn. When I had the affected engine on the pressure gauges it did however build normal pressure.

  5. Member
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    #5
    possible scored cyl or reed problem in that compressor
    .................................................. ...the scariest thing in life is the unknown ...................................

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    #6
    Is this best course of action a replacement compressor if that's the case? The compressor is the aluminum colored one not the updated black compressor that apparently is a newer version. Would I also need to send out rails and injectors? The engine runs well at any RPM above idle at this point. The only evidence of an issue is the miss at idle.
    Last edited by zx6rr33; 06-14-2021 at 08:16 AM.

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    #7
    yes sir , good idea too have them looked at
    .................................................. ...the scariest thing in life is the unknown ...................................

  8. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #8
    Believe I spoke with you on the phone today regarding this engine... if so, you had a new compressor and return oil line filters ready to go on.

    But you were going to pull apart the old one just to identify why you could turn the pulley clockwise over and over and over.

    The Serial Number you provided is also missing a digit.


    Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
    Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
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    #9
    Update, changed the compressor, as per your instruction Don I removed the hose at the top of the starboard rail and found no evidence of aluminum or metal shavings. I’ve not yet disassembled the fact or compressor but I suspect it is simply worn. The engine is quieter with the new compressor but still has a miss. I also changed the tracker valve with no improvement. I’ve swapped the coils from a known good engine as well and no difference.

    When I start pulling plugs off injectors to try and narrow down which cylinders are the issue I found something interesting. All injectors will lower rpm when disconnected but when I disconnect one on the port rail the miss goes away. If I disconnect an injector on the starboard rail the miss stays. Thoughts?

  10. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #10
    WHICH injector on the port rail did you disconnect (specifically)?

    I would be inspecting the spark plug, wire, and REEDS on that cylinder.


    Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
    Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
    Mercury Parts, Mercury Outboards, Smartcraft & Accessories, Injector Service, TDR Reeds- BBC Sponsor

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    #11
    That’s kind of the issue. I continues to miss if I pull the fuel injector wire off of any cylinder on that side.

  12. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #12
    Quote Originally Posted by zx6rr33 View Post
    All injectors will lower rpm when disconnected but when I disconnect one on the port rail the miss goes away.
    Quote Originally Posted by zx6rr33
    That’s kind of the issue. I continues to miss if I pull the fuel injector wire off of any cylinder on that side.
    So which above statement is a correct representation?


    Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
    Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
    Mercury Parts, Mercury Outboards, Smartcraft & Accessories, Injector Service, TDR Reeds- BBC Sponsor

  13. Member
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    #13
    Sorry I didn’t mean to be confusing.

    When I pull any plug off any injector plug port or starboard rail the idle slows on the engine. However, when I pull any of the injector plugs on the starboard rail the lowered idle also has a miss. If I pull any of the injector plugs on the port rail the idle slows and the miss disappears.

    Are there any grounds I should be checking on the injector wiring harness?

  14. Member lpugh's Avatar
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    #14
    I would pull the reed blocks for inspection
    Cylinder contribution tests on a motor that is not running correctly can sometime be deceptive and confusing
    Might rerun that test with the throttle set for 1000 RPM and document the actual RPM drop on each cylinder
    Thank You Leon Pugh

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    #15
    Okay I’ll do both of those things and report back

  16. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #16
    I'm leaning toward a reed problem... be sure you pull the entire air handler assembly so they can be viewed from the BACK side.


    Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
    Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
    Mercury Parts, Mercury Outboards, Smartcraft & Accessories, Injector Service, TDR Reeds- BBC Sponsor

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    #17
    Okay I pulled the air handler and reed plate out completely today. All of the reed petals are intact with no chips or splits. Some of the reeds seem to have a small gap on the leading edge of the petal between the block and the reed. The alignment with the blocks and the reeds look perfect though.

    When I fired it back up after running the oil prime sequence the engine fired and idled perfectly for two or three minutes then started it’s usual missing again. I’m a little frustrated.

    What’s next?

  18. Member lpugh's Avatar
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    #18
    Since it ran well for a while, I would range all the sensor when warmed up
    Next I would switch the complete fuel rails including the direct injectors with the other motor
    Consider using bore scope to check for a turned sleeve, Joe has mentioned rough idle as well as overall bad performance with this problem
    Last edited by lpugh; 06-20-2021 at 08:16 PM.
    Thank You Leon Pugh

  19. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #19
    Cold engine will be on a rich MAP table for fuel delivery. Once the engine reaches approximately 100F the MAP table multipliers (big ones) drop off.

    HOWEVER: If one of the banks is not coming up to temp properly, that bank will be too rich. This particular vintage had what might perceived as a notable "miss" when one bank was too cold.

    Might want to double check that both banks are reaching 120F or higher AT IDLE.


    Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
    Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
    Mercury Parts, Mercury Outboards, Smartcraft & Accessories, Injector Service, TDR Reeds- BBC Sponsor

  20. Member
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    #20
    Thank you guys, I will check the temp today (infrared gun and scan tool is best for this?) and if that doesn’t appear to be the issue I’ll swap the fuel systems and see if that cures the problem. Thank you very much for steering me in the right direction with this.
    Last edited by zx6rr33; 06-21-2021 at 06:11 AM.

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