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Compression and Vaccum Questions?

Discussion in 'The Garage' started by johnathan, Sep 19, 2006.

  1. johnathan

    johnathan 1/2 ton status

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    We did a compression & vaccum test on my dads old 350.The compression was at 170 psi. on most cylinders,165 psi. ,160 psi. Is this too high? I know in my Haynes manual it says 150 psi.


    The vaccum was low and erratic.I'm guessing a vaccum leak.Any advice?
     
  2. BadBob

    BadBob 1/2 ton status

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    Compression's excellent. I'd start chasing down vacuum lines.
     
  3. 78Suburban

    78Suburban 1/2 ton status

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    Take carb cleaner and spray it all around your intake, carb base plate gasket, all vaccume lines.. when you hear the idle speed up, there's your vaccume leak.

    Also, put your hand over the carb with the engine running and if the idle speeds up, you probably have a vaccume leak.
     
  4. ssped

    ssped 1/2 ton status GMOTM Winner

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    if you checked it with a vaccum guage and it fluctuates it would mean an intake valve is not closing all the way.
     
  5. 85mudblazin

    85mudblazin 1/2 ton status

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    Also all your compression numbers should be close to each other. 170 and 160 is very bad. I think you want each cylinder within 3 or so PSI of each other.
     
  6. K5er4Life

    K5er4Life 1/2 ton status

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    thats not bad at all, thats only 6% difference in cylinder pressure. Thats quite good actually for a motor that probably has a ton of miles on it. I think most manuals say that there should be no more than a 10% difference in compression.
     
  7. 79BRUISER

    79BRUISER 1/2 ton status

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    so if the manual says it should b e 150 the 90 psi i got on my 350 the other day is poor? does that mean bad rings, rebuild. it has no power in the low rpm range, 3000 rpm and under but in the upper rpm range it pulls runns better. thats 90 on each cyl.
     
  8. diesel4me

    diesel4me 1 ton status Premium Member

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    not that important..

    Its not so much the amount of compression,but that all the cylinders have close to the same amount..old engines have carbon build up that reduces the chamber size and increases compression..170 lbs isn't unusual..I have owned engines with only 100 lbs compression that ran great,and used no oil..

    However,90 lbs IS low,but if ALL the cylinders have close to 90 lbs,its probably a combo of worn rings and cylinders,but first be sure the valves are adjusted properly,and the timing chain isn't about to jump--late valve timing will reduce compression too!..a "hotter" cam than stock can reduce compression at cranking speed tooo,due to wide valve opening overlap..:crazy:
     
  9. 79BRUISER

    79BRUISER 1/2 ton status

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    Thanks, Ive got a friend that builds his own engines comming over to look at it.
     

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