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  1. #1
    Join Date
    May 2009
    Grant Valkaria

    Compression concerns???

    I'm looking at a 2005 Ranger Z21 in another state with a Mercury 250 four stroke on it, sorry I don't have serial number yet, salesman is sending tomorrow. He says the hours on the motor are 230 hours. The compression are as follows:
    #1 cylinder =170
    #2 cylinder =190
    #3 cylinder =175
    #4 cylinder =190
    #5 cylinder =190
    #6 cylinder =185
    Should this fluctuation in cylinder compression be something to worry about? I haven't never owned a for stroke so not for sure? Thanks

  3. Member
    Join Date
    Jul 2016
    in cars i like to see less than 10% variation between cylinders. here that would be 19 psi from your highest 190 psi reading. your lowest is 170. so that's Very slightly over the 10%. Being that it's an outboard it may be slightly different acceptable range.

    If it was me i wouldn't be too overly concerned, but there are 2 cylinders that's lower than the rest. #1 #3... Makes me wonder about them.

    I assume that it's the verado?

  4. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
    Join Date
    Apr 2008
    Greenville, SC (US)
    I'd recommend a LEAKDOWN test. Far more accurate, especially on these Verado Models (it's difficult to have the throttle plate open consistently to the WOT position, as it's PCM controlled).

    Dual Mercury Master Technician- for Mercury Outboards, Racing & Mercruiser Sterndrives in Greenville, SC.
    Still consider myself a "Marine Apprentice" after 40 years (learn something new every day).
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  5. #4
    According to data in the Mercury Verado SM your motor appears to be just fine from a compression standpoint. Using Mercury's formula, and based on the highest cylinder in the motor you are looking at (190 psi), the lowest cylinder should be no less than 161.5 psi. The lowest cylinder you mention is 170.

  6. #5
    Mercury Marine Customer Support told me they had no objection to my posting this snippet from the service manual.