Ran a compression test. Uh oh.

Ran a compression test. Uh oh.

BillyB

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85 CJ7 258 T5 Dana300 Dana30 AMC20
I've known this engine isn't happy. But the extent of its unhappyness is beginning to show.

1 - 0
2 - 60
3 - 61
4 - 60
5 - 54
6 - 40

Wow. Ran that 1st one a few times. All are average of 3.

Anything I can do short term before I start looking into the complete rebuild?

Leak down test is next on the list to get the full picture. But man....

Todd
 
Way slow timing could also give low readings.
 
Maybe rings maybe valves. There is the old test of putting a tablespoon of oil in the cylinder to see if that helps compression. If so it's the rings. If not its valves.
You've got a zero on one cylinder. I bet a valve is stuck. You should be able to find out if you remove the valve cover.


Sure a leak down test is more accurate than a compression test but if you've got zero on one cylinder we already know enough.

EDIT: How many miles on this engine? If it is the valves you could remove the head and get a valve job. On the other hand if the engine has a lot of miles it may not be long before the rest of the engine will need a rebuild.
 
Last edited:
There is a slight chance that your cam is retarded due to a bad timing chain but the zsero reading sounds like a stuck valve, bent valve or a shot intake lobe on the cam. I would gamble and say you need a cam shaft and new timing chain. It is very rare to see all cylinders with equally low compression. Go and do the oil in the cylinder test, if the numbers stay the same then either measure your valve lift or at least pull the cover and watch the rockers actuate to see if they appear equal. If the compression does not go up from the oil test then buy a new cam, lifters and chain.
 

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