compression test

Discussion in 'KJ General Discussion' started by offrovering, Apr 15, 2009.

  1. Apr 15, 2009 at 8:38 AM #1
    offrovering

    offrovering Full Access Member

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    has anyone done a compression test on a newer liberty engine, and then also on an older engine?

    I have to find some issue with my other vehicle and figured I would compression and vacuum test the liberty while I am at it. I have no baseline to go off of though, so just curious.

    I know 10% difference, but still like to have a baseline to know where these engines start.
     
  2. Apr 15, 2009 at 9:00 AM #2
    Powerslave

    Powerslave Banned

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  3. Apr 15, 2009 at 9:07 AM #3
    JeepJeepster

    JeepJeepster Full Access Member

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    He wants the PSI that each piston should have when doing a compression test, not the ratio.

    Im not sure what it is.. Could be around 150psi...
     
  4. Apr 15, 2009 at 8:21 PM #4
    tjkj2002

    tjkj2002 Full Access Member

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    '02-'04 3.7 has 9.1:1 compression ratio,cylinder pressure should be between 170psi-225psi with no more then 25% difference,right from the '02,'03,and '04 FSM.

    '05+ 3.7 has 9.6:1 compression ratio,cylinder pressure is the same which is 170psi-225psi with no more then 25% difference,right from the '05,'06,'07,'08(KK) FSM.
     
  5. Mar 25, 2018 at 5:55 PM #5
    PunkChachi

    PunkChachi Full Access Member

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    I’ve never done a compression test before but it seems pretty straight forward. I am having constant misfires on cyl 2 (p0302), did the “dollar bill test” against my tailpipe and it was getting pushed/pulled to indicate possible burnt valve. I have a compression tester but do I need to test each cylinder and compare the results or Can I just test #2 and if it is outside of the above stated specs that is how I know it is bad? Also, is there a way to tell if it is a burnt valve vs sticking valve?
     
  6. Mar 26, 2018 at 1:52 PM #6
    04Liberty

    04Liberty Full Access Member

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    To do it properly you need to do all of them. You can certainly do that one just to see if it's in spec, but you want to be able to compare it to the others (healthy engine they should be all relatively close to each other).
     
  7. Mar 26, 2018 at 5:31 PM #7
    jja

    jja Full Access Member

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    I have been measuring mine at each spark plugs change. It is in the 150-160 psi range with the engine at room temperature.
     
  8. Mar 26, 2018 at 8:43 PM #8
    PunkChachi

    PunkChachi Full Access Member

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    So I did all of them and all but one cylinder #4 (which I have gotten misfire codes for before) were between 178-180 psi. #4 Sat down at 150 psi :( I added a few drops of oil and it jumped up to 180 psi.

    My big question here is, can a stripped spark plug hole cause low compression? Ever since I bought the Liberty I have always had a tough time getting the #4 spark plug in. When I went to screw in the Compression gauge it would only screw in less than one turn. Could this be the cause of my low compression on that cylinder and is there a way to test it to confirm before I do some major teardown on my engine pointlessly?