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Cylinder Compression

Discussion in 'Early CJ5 and CJ6 Tech' started by 9thlifer, May 4, 2017.

  1. May 4, 2017
    9thlifer

    9thlifer New Member

    Texas
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    May 4, 2017
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    I have a 59 CJ5 with the F134 engine. Anybody know what the cylinder compression should be? I've got the jeep running again after sitting about 5 years and it's blowing light grey smoke. A lot of it. I've cleaned the gas tank, oil pan, added new ignition components, etc. Am trying to figure out if it's rings, head gasket, fuel, or what, and I need the numbers for decent engine compression. Thanks in advance!
     
  2. May 4, 2017
    haighfam

    haighfam Member

    Paradise, CA
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    Specs say #120-#130. Did you put any oil on the cylinders for storage? I did on mine and it took about 2 hours to get it all burnt out of the exhaust system.
     
  3. May 4, 2017
    9thlifer

    9thlifer New Member

    Texas
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    Interesting, I think I did spray some oil or PB Blaster or something at one time. I would occasionally go out and turn the engine by hand to keep it loose. My readings were from 112, 115, 115, 108 on kinda warm engine with throttle open. Am I thinking right that smoke from excess gas would be black, oil would be blue, and water/antifreeze would be white?
     
  4. May 4, 2017
    PeteL

    PeteL Member Sponsor 2019 Sponsor

    Hills of NH
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    I'd be happy with anything over 100# on a used F-head - but the critical point is that the four cylinders should all be about the same
     
    timgr likes this.
  5. May 4, 2017
    timgr

    timgr Jeepin' Nerd Sponsor 2019 Sponsor

    Medford Mass USA
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    Yep. Even is where it's at.
     
  6. May 5, 2017
    Rick Whitson

    Rick Whitson Detroit Area Sponsor 2019 Sponsor

    I live South of...
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    Did you try squirting a little oil in each cylinder and checking it again? That will seal the rings, if it goes up the rings are the reason for the difference, if it does not the valves are the reason for the difference. Good luck what ever you do.
     
  7. May 5, 2017
    Howard Eisenhauer

    Howard Eisenhauer Super Moderator Staff Member Sponsor

    Tantallon, Nova...
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    User xz3ltt posted this up way back in 'aught-four :


    When performing a compression test, your be looking at variations "within" each cylinder and also "across" all cylinders..

    A: Do the test AFTER the engine has been run at normal operating temp.

    B: Per cylinder, record the first and 4th stroke of the needle on the compression guage
    (you have to look at the guage for the 1st stroke of the needle...
    the needle on the guage will stay on the 4th stroke until you release the valve on the guage).


    The readings...

    Typically want less than a 10% variation "within" and "across" the cylinders. Service manual will tell you want
    compression should be within the cylinder (believe less than 80 is typically not good)

    Within each cylinder, if the first stroke is more than 10psi lower than the last/4th stroke, add oil to the
    cylinder and retry. If the 1st stroke is better on the retest, there's typically a ring issue. If no change, look for valve issues.

    If readings are low and identical on two cylinders next to each other, check for head gasket issues.

    A vacuum guage is also good for checking the engines health (when running) as is a cylinder leak down test.


    Your readings don't look that bad to me- not factory/rebuild fresh but reasonably good.


    FWIW I built myself a leakdown tester & I can not emphasize enough what a great tool it is for checking compression issues:)

    H.
     
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  8. May 6, 2017
    Rick Whitson

    Rick Whitson Detroit Area Sponsor 2019 Sponsor

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    What Howard said, I wouldn't do anything yet, just enjoy it like it is. Good Luck.
     
  9. May 6, 2017
    1960willyscj5

    1960willyscj5 Well-Known Member

    Mesa, Arizona
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    112, 115, 115, 108
    That is actually quite decent for an engine that has been sitting in a barn for 5 years.

    Bet if you run the JEEP around for a while those numbers would come up.
     
  10. May 18, 2017
    Greevesman

    Greevesman Sponsor Sponsor

    Napa, Ca
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    I just borrowed a compression tester. I was told this F134 has been rebuilt by the owner maybe 8 years ago. It is a strong runner so I was curious.
    1=145#, 2=145#, 3= 141#, 4=150#. I did not do the add oil test. Quite a bit higher than the FSM spec. I wonder if they milled the head?
    Oh well, it runs good, No detonation.
     
  11. May 19, 2017
    timgr

    timgr Jeepin' Nerd Sponsor 2019 Sponsor

    Medford Mass USA
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    Compression will go up a little when the cylinders are bored oversize - no head planing needed. You end up with a slightly larger displacement for the same combustion chamber size. Also, a generic compression gauge may not be very accurate, but it's likely precise if your technique is consistent. The lower the variation from cylinder to cylinder and with repetition, the better - the variation you posted is quite low. Adding oil could make them go up a little, but it's mostly pointless with numbers like that. The oil test would tell you something about the rings if the compression were lower and more variable.
     
    Last edited: May 19, 2017
  12. May 19, 2017
    Mark Wahlster

    Mark Wahlster Member

    Silverton, OR
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    With the octane gas we have no you would have to be running some pretty wildly advanced timing to get any predetonation These engines at 6.48 to 1 and 7-1 were designed for 68 Octane fuel the most volatile stuff you can find around here is 87 Octane. Which will easily support 9-1 if not 9.5-1 compression with timing advanced some over stock.
     

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