Low compression in one cylinder :(

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Aug 10, 2016 | 07:27 PM

Ethan1997

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Low compression in one cylinder :(

My '88 pickup with the 351w seems to have developed an issue with compression in cylinder #5. The others have about 130 cold and the motor runs awesome except it shakes a little due to the unbalanced cylinder. I'm wondering if its worth putting new rings in and maybe putting a mild cam in it while its out. It has 145k on it. Just looking for opinions on whether its worth it or should I get a used block. Thanks

Aug 10, 2016 | 07:31 PM

85e150

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Did you do it dry and then again with a little oil in each cylinder?

Could be a valve issue if you are lucky.

Aug 10, 2016 | 08:33 PM

roboto65

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From: Conroe,TX

I would do a leakdown test or like I do airhose to cylinder on tdc both valves closed and listen to where the air is coming out exhaust or TB 130 is not bad cold dry.
I am thinking a bad valve.

Aug 10, 2016 | 09:58 PM

Ethan1997

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Thanks guys for the quick replies! The test I did was cold and dry so I assume the other cylinders are pretty golden at 130psi straight across the board. #5 was at 50psi so I am going to try a wet test and then airing up the cylinder. The problem is that the spark plug in #5 gets soaking wet with oil almost instantly after start-up so I am leaning towards the rings but we'll see and I will report back!