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one cylinder no compression?

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Old 08-09-2007, 10:06 PM
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Default one cylinder no compression?

trying to help my buddy he, he has a miss at about 4500 rpm so we pulled the plugs everything looked good, so we did a compression test and on number 7 we are getting erratic pressure sometimes we get 120psi and 90 psi and then it will crank for a while and have no compression, all the rest of the cylinders have 145psi. no miss down low and it sounds great? can it be a valve? i dont understand how we can sometimes have compression. its a chevy 572 big block
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Old 08-09-2007, 10:18 PM
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This happened to me and it was a broken valve spring.Pull the valve cover to make sure. You may get lucky and be able to fix with little disassembly. Mike
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Old 08-10-2007, 07:55 AM
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Quote:
Originally Posted by Michael Garibay View Post
This happened to me and it was a broken valve spring.Pull the valve cover to make sure. You may get lucky and be able to fix with little disassembly. Mike
Man, I never get that lucky.
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Old 08-10-2007, 07:56 AM
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Man, I never get that lucky.

Aint that the truth!!!!!!
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Old 08-10-2007, 09:04 AM
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Every time I have seen a broken or weak valve spring it will POP (backfire) at higher RPMS. The valve dosen't get closed in time because of the spring.
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Old 08-10-2007, 09:09 AM
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No evidence of moisture in #7?

Sounds like a valve sticking. Could be a weak or broken spring as mentioned.
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Old 08-10-2007, 09:45 AM
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I snapped a rocker stud on the exhaust side once...broke a rocker arm in half once...seized a lifter once, etc, etc, etc.
Let us know what you find.

Last edited by Back4More; 08-10-2007 at 09:49 AM.
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Old 08-10-2007, 10:39 PM
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well couldn't any cracks in the spring for the exhaust valve but i replaced it anyway and now all is fine. 150psi of compression!
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Old 08-11-2007, 03:55 AM
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Good news. It was weak, but not weak enough to be felt at lower rpms or cause a backfire. Your buddy should consider himself lucky.
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Old 08-11-2007, 02:32 PM
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Congrats on the simple repair but I would be concerned with the rest of the springs so stay alert.Mike
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