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Mercruiser 5.7, Uneven compression

Cyl 1 (160) Cyl 3 ( this is one of the two that had the manifold water leak 115 ) Cyl 5 ( the other one that had water 125) Cyl 7 (170) Cyl 2 (155) Cyl 4 (160) Cyl 6 (125) Cyl 8 (160) Any sugestions as to how I should move forward would be greatly appreciated. The motor is out of the boat on a stand, so I can go in any direction I need to go.
 

makomark

Silver Medal Contributor
compression test-wise, I'd say the engine is at the end of its service life...not knowing any of the history its hard to say...you can buy brand-new and have it installed, at one end, compared to stripping it down and rebuilding it yourself...
 

louc

Regular Contributor
Ideally you'd do a leak down test to see where you're losing compression, a good Chevrolet small block should be at least at 150 psi, on my 4.3 V6 it tests out between 160-170 psi. Just guessing the cyls that had water could have sticky exhaust valves, but it could also be corrosion damage to the cyl wall. Depending on what you find, you might be able to get it back to where it should be with a top end overhaul assuming that the low compression test results are more so due to sticky valves than cyl wear or ring wear
What was the oil pressure and oil consumption like when you were running it? Any odd noises?
 
Ideally you'd do a leak down test to see where you're losing compression, a good Chevrolet small block should be at least at 150 psi, on my 4.3 V6 it tests out between 160-170 psi. Just guessing the cyls that had water could have sticky exhaust valves, but it could also be corrosion damage to the cyl wall. Depending on what you find, you might be able to get it back to where it should be with a top end overhaul assuming that the low compression test results are more so due to sticky valves than cyl wear or ring wear
What was the oil pressure and oil consumption like when you were running it? Any odd noises?
It ran beautiful, so I bought. It always ran smooth with no heavy oil consumption. One day the outdrive froze up. A mechanic didnt realize it was the outdrive and he was pulling plugs to try and turn the motor over. That's when he noticed water in the cylinder, and the leak between the manifold and the riser was pretty evident. I wanted to replace the gimbal housing so I pulled the motor and figured, while its out, Ill deal with the water issue. That's where the compression test came in.
 
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