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compression test results - disappointing

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by 90K5, Mar 30, 2003.

  1. 90K5

    90K5 1/2 ton status

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    did a compression test today, hoping to shed some light on my surging problem. results not so good...
    #1 - 75
    #2 - 100
    #3 - 100
    #4 - 80
    #5 - 100
    #6 - 100
    #7 - 80
    #8 - 95
    I was hoping the numbers would be a little higher...as it stands, i think there is something internal causing my problem, but with the numbers so low, i dont know if I want to pour any time and money into a dying engine..it has 175K on it, but has been maintained very well. I know these things last a long time, but looking for suggestions...are these really that low? The varying numbers have me a little concerned too, making me think this engine isnt going to last as long as i was expecting...dont know whats wrong with it yet, its not the MAP sensor, intake gaskets, coolant temp sensor, IAC motor, plugs, cap, rotor, EGR, O2 sensor, knock sensor, and probably some more things i forgot...the only things really left are the computer and internal parts...dont want to throw a new engine in there till i know its not the computer, and dont want to replace the engine if its not the problem...thinkin i might have a burnt valve, but still cant be sure...anyone have any thoughts on this? thanks
     
  2. 90K5

    90K5 1/2 ton status

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    oh yeah...as far as i can tell its not the timing, but i guess the balancer could have slipped and the timing marks moved...checked it today, couldnt really tell, but it seemed about right
     
  3. Miller

    Miller Registered Member

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    valves were my first thought and pretty much my only thought. if youve got the time to have the truck down for a few days, yank the heads off and have buddy in the machine shop. refresh the heads.

    bet the compression readings would be more consistent 100, but not much higher.
     
  4. 90K5

    90K5 1/2 ton status

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    yeah, i really need to just tear into it i think...problem is i had to leave it back home, fly out to school. Left it at a mechanice friend's shop, hes going to look at it when they arent busy so the price doesnt go too high. Dont really want to go back every weekend till i fix it, but need transportation.
     
  5. HarryH3

    HarryH3 1 ton status Author

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    Those numbers pretty much suck, especially for a test done near sea level. /forums/images/graemlins/frown.gif With that many miles, it's possible that the timing chain and gears are worn out and it jumped a tooth on the timing gear. That will really mess up the compression because then the valves are opening and closing at the wrong times.
     
  6. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    A burnt valve would leave you with zero in one cylinder. With 175K on the clock I'd say rings and normal wear...

    It doesn't look like you have any two low readings next to each other...assuming you listed the compression by cylinder numbers. Sometimes two low readings next to each other indicate a bad head gasket...

    Rene
     
  7. 90K5

    90K5 1/2 ton status

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    if it is the timing chain, and i replaced it, woould the compression still be messed up? Since its so low, and has so many miles, the minute i find out its not the computer messing it up im ordering a crate motor. I just dont want to throw a new engine on there only to have the same problems...what still makes me think its computer related is that it runs fine until it is hot, then starts surging...unitl i floor it, then it all evens out again
     
  8. limey

    limey 1/2 ton status

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    i take it that was a dry test. now put a few drops of oil in the spark plug holes and try it again if the numbers go up its you rings if it stays the same its your valves.
     
  9. 90K5

    90K5 1/2 ton status

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    hmmmm...disnt think of that, good call
     
  10. 90K5

    90K5 1/2 ton status

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    well its still acting like a vacuum leak, I think either the EGR or TBI to intake gasket is bad....the TB kit only came with one gasket, and i have a spacer, so i need 2. Replaced the worst one, but neither one looked too bad, and ive been reusing the EGR one since it didnt look too bad, but now that i think of it, it looked sort of leak-prone. We will see.
     
  11. xnoahx

    xnoahx 1/2 ton status

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    what are "good" numbers to get from a compresstion test for a 350?
     
  12. Pure Insanity

    Pure Insanity 1/2 ton status

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    175K, Id say youve got your moneys worth outta it. Start thinking rebuild or crate motor. As a rule, 10% difference in compression is considered OK. Your at 25%, as well as just being low in general.

    Its just gettin tired.
     
  13. DesertDueler

    DesertDueler 1/2 ton status

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    How do your plugs look, and does it burn any oil? If oil builds up on the plugs it will casue it to run rough because it isnt getting the spark you need, but will go away once you start acc. A IAC motor starting to go bad will also cause a surging problem. Did you replace the IAC, or just test it? Also a cam going flat would cause a surge.As for good reading, you should get at least 150 psi out of each cylinder without having to put any oil in the cylinders.
     
  14. 90K5

    90K5 1/2 ton status

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    It burns a little oil, but the amount kind of varies...it burns 10w30 like nothing, but usually about a quart of 20w50 per 3000 miles. smokes a little when i start it if its been sitting for a day or two. plugs were a week old when i first looked at them again...they looked like they had been in there a little longer than that, but they really werent that bad. Last weekend, after i changed the intake gaskets and rebuilt the TB, pulled the plugs to test compression and they were fouled bad...it had been running way rich, so bad it was stalling - i think i missed something when i rebuilt the TB, cause it wasnt running rich before. IAC motor i replaced, which helped it idle smoother, but the surging didnt go away. We unplugged it and it made no difference when troubleshooting.
     

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