Mercury Optimax Compression Test

  • Matt Vogel
    Posts: 151
    August 5, 2022 at 7:43 am #2139612

    I am prepping to sell my boat so I had a local dealer do a hour pull and compression test. I got a call last night with results and they said the motor runs great and compression is dead nuts identical across the board but lower than he expected and he is not sure why. He thought maybe if the battery was almost dead that could contribute. He will be running another test but I am wondering if there is anything he could be doing wrong that is causing the low compression. I know the important thing is that they are consistent but I am also curious. I have read online that the Mercury Opti’s compression numbers are typically lower than others due to the compression ratios, but still usually 80s-90s. If he didn’t let the motor warm up first could that contribute? Anyone have any experience with this at all?

    P.S. – Anyone looking to buy a boat? wink lol

    Iowaboy1
    Posts: 3831
    August 5, 2022 at 10:58 am #2139662

    He thought maybe a bad battery would cause low readings????
    SHEESH!!!!
    Any motor getting a compression test HAS to have a fully charged full capacity battery or you will never get an accurate test period.

    The Optimax motor sounds weird when starting anyway and I have had a few customers say its still not cranking over like it should even after they installed a new battery.
    After verifying the cables and connections are corrosion free I removed the starter and disassembled it to find a corroded ground brush connection where it mounts to the starter brush holder.
    Clean everything up, install new brushes if needed and that starter will spin that Opti like no tomorrow.

    TheFamousGrouse
    St. Paul, MN
    Posts: 11838
    August 5, 2022 at 12:06 pm #2139681
    Matt Vogel wrote:

    He thought maybe if the battery was almost dead that could contribute. He will be running another test but I am wondering if there is anything he could be doing wrong that is causing the low compression.

    If he thought maybe a low battery could cause an inaccurate compression reading, but he wasn’t really sure…

    Yeah, find a different mechanic. Sounds like a freaking gong show at that shop.

    Drizzy Musky
    Duluth
    Posts: 258
    August 5, 2022 at 4:05 pm #2139724

    Hope the mechanic just isnt very good with words.

    I agree with Iowa and Grouse.

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