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well boys i think i may of found my lil problem...

Discussion in 'The Garage' started by chvymudin3388, Feb 4, 2007.

  1. chvymudin3388

    chvymudin3388 1/2 ton status

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    no wonder i got bad blow by, i ran a comphression test and all cylinders read in a good 120 psi except for nubmer 3... it reads like 10 if that. so what you guys think? washed out the side of the piston? broke ring? hole in the piston?
     
  2. HarryH3

    HarryH3 1 ton status Author

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    Only way to find out is to take it back apart. :( Assuming that the valves on that cylinder are closing... ;)
     
  3. Chevy305

    Chevy305 6 Lug 14bsf Status Premium Member GMOTM Winner

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    Ya pull the valve cover and make sure the valves are opening and closing fully.
     
  4. sweetk30

    sweetk30 professional hooker Premium Member

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    x2:rolleyes:
     
  5. dontoe

    dontoe 3/4 ton status GMOTM Winner

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    Yeah, might just be a valve spring..........:wink1:
     
  6. Confedneck79K30

    Confedneck79K30 3/4 ton status

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    120psi is good compression? does compression vary depending on engine compression ratio or piston stroke? or what? not to hijack...
     
  7. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    Depends on a lot of things from cam overlap, cam timing, static CR, and cranking speed (just to name a few variables)

    Rene
     
  8. HarryH3

    HarryH3 1 ton status Author

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    Altitude also affects measured compression. As you gain elevation, atmospheric pressure drops. Less air going into the cylinder means less pressure when the piston tops out. :(
     
  9. Confedneck79K30

    Confedneck79K30 3/4 ton status

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    ty gents...
     
  10. HarryH3

    HarryH3 1 ton status Author

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    For a ball park measurement at sea level, compression should be somewhere around 14.7 (atmospheric pressure at sea level) times the compression ratio. So for an 8.5:1 compression ratio, you'll see somewhere around 14.7*8.5=124.95 PSI. It's just a ballpark figure, as valve timing affects it, but it should get you within 10 PSI or so on a stock, tight engine.

    Cam selection plays a role and more radical cams will let some pressure bleed off at cranking speeds due to longer valve overlap duration.
     
  11. chvymudin3388

    chvymudin3388 1/2 ton status

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    well thats what i dont get is it seems to idle fine, and if it was a spring wouldnt i t like misfire or backfire or something? plus its building pressure in my motor enough to pop out the PCV out of the valve cover and no its not plugged.
     
  12. HarryH3

    HarryH3 1 ton status Author

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    Most likely. The blowby problem really points to something bad wrong inside the engine. :( Pressure is getting past the combustion champer and pressurizing the crankcase. A stuck valve wouldn't cause that. :(
     
  13. chvymudin3388

    chvymudin3388 1/2 ton status

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    see thats why i dont think its a valve. thats why i think it a ring or piston
     

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