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  1. #1
    Join Date
    Mar 2012
    Posts
    13

    Default Compression Tests

    What would be an acceptable compression variance (+/- psi) for a 1989 Mariah? thanks!

  2. #2
    Join Date
    May 2006
    Location
    Nashville
    Posts
    2,122

    Default

    If I remember right, and I probably don't, I wanna say the range you would like to be in is 120-140 PSI for an engine of that age.

    It's 8.5:1 ratio.

  3. #3
    Join Date
    Sep 2011
    Location
    GA
    Posts
    343

    Default

    I think you want to stay within 10% but not sure it works the same on a standard motor versus a 2stroke.....I always thought it was compared different

  4. #4
    Join Date
    Mar 2012
    Location
    Northern IL
    Posts
    28

    Default

    I am looking at a 1990 comp right now with a questionable variance myself. The PCM manual does't say what the compression psi
    should be but it does say that if the lowest reading is within 75% of the highest it is ok. That seems like a lot to me. The only
    other info I found was that allegedly these motors can still run good at as low as 90 psi. The boat that I'm looking at is between
    100-125 psi. Still waiting to test it on the water.....

  5. #5
    Join Date
    Jul 2007
    Location
    Flowery Branch Georgia
    Posts
    1,675

    Default

    Those readings are not bad. Can you post each cylinders reading and how the test was run? How many hours are on the engine?

  6. #6
    Join Date
    Mar 2012
    Posts
    13

    Default

    the 1989 Mariah I'm interested in reads;

    Compression is:
    #1 145
    #2 150
    #3 150
    #4 145
    #5 135
    #6 145
    #7 145
    #8 145

    All within 10%. Not sure why they are so high. It has the 330 hp PCM. Higher HP = Higher PSI??? idk.

  7. #7
    Join Date
    Mar 2010
    Location
    Windsor, Ontario
    Posts
    1,103

    Default

    The 1989 Mariah numbers look great to me.

    Usually if you have a cylinder with a low reading add some oil to it. If the number jumps up to normal readings the issue lies with tired or worn rings. No change then the issue is the top end, head or head gaskets. Anything over 100 is good in my eyes, unless they are all in that ball park. I have seen many engines with a tired hole which run just fine.

    2003 Supra Launch " Gravity Games Edition"
    Dodge ram CTD tow vehicles....

  8. #8
    Join Date
    Aug 2009
    Location
    London, Ontario, Canada
    Posts
    1,004

    Default

    those numbers look good, # 5 has the greatest diff and its only 10% the rest are virtually the same
    2009 21v Worlds 340 Cat
    run your engine after you change the oil
    Doug

  9. #9
    Join Date
    Jul 2011
    Location
    fort worth , tx
    Posts
    830

    Default

    i would agree , 125 psi or more is fresh with a stock bore , no worries there .... i always like to check valve lash and make sure im good there before compression comes on the
    plate

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