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  1. #1
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    1999 200 EFI compression test result interpretation

    Serial #0G939607
    Test was done on warm motor with all plugs removed and throttle wide open.

    Port:
    115
    115
    111

    Starboard:
    118
    122
    119

    Can anyone give me an idea why the port side cylinders are lower than the starboard? The port side head is cooler to the touch when running on muffs but I’ve read this is normal. Any ideas? And should I be concerned about the 111 as Mercury states the specs for this motor are 110 - 130? I tested the port side first so battery charge would not be the cause.
    Last edited by Crooked stick; 11-02-2022 at 09:48 PM.

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    #2
    You did it right.Mine is a year older with basically the same results.I would consider it good if everything else checks ok. Don't know why port side is cooler. Waiting for the answer...

  3. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #3
    Not any real concern. The difference is likely a slight variation in the cylinder head (cc measurement) castings.


    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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    #4
    So I worked on the boat over the weekend and I found the cause of a misfire that the boat has had for probably the last couple years or so. When running on muffs I discovered the port side thermostat housing dripping water down onto the port side plugs - the same bank of cylinders that have the slightly lower compression. So after discovering this it makes me wonder if the intermittent misfire on these cylinders would be the cause for the lower compression? Could the misfire have caused the cylinders to wash and score?

  5. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #5
    Wash: Yes. Score.... less likely but not impossible. But you might retest again after 10-20 hours runtime just to see what difference you note.


    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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    #6
    Quote Originally Posted by EuropeanAM View Post
    Wash: Yes. Score.... less likely but not impossible. But you might retest again after 10-20 hours runtime just to see what difference you note.
    Do you say it’s unlikely to score a cylinder due to the oil in the gas being a 2sttoke?

  7. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #7
    Yes- less likely (but not impossible).


    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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    #8
    I’ll get a few more hours on it and check it again with fingers crossed..

  9. Member lpugh's Avatar
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    #9
    Quote Originally Posted by Crooked stick View Post
    Do you say it’s unlikely to score a cylinder due to the oil in the gas being a 2sttoke?
    I have seen several of that vintage with compression very similar to what you have, it is most likely a slight difference in deck height of the block or the combustion chambers. Jay Smith says this was very common issue back in those days to have about 5 to 10 psi difference side to side. I had that was that way until I milled and CC,d the heads

    You can perform a cylinder leak down test on both sides to determine if sealing is the same
    Thank You Leon Pugh