Low compression 1 cylinder 88 Mercury 60HP

[SMOKNZ]

I have been looking into my 1988 60 HP Mercury outboard as to why it has lower compression in one cylinder. The top cylinder is at 100 psi and the lower 2 are at 127psi. I popped off the cover to the top of the cylinder water jacket to look around for cracks and I found none. I looked thru the spark plug hole to see if I could see anything wrong with the piston. I didn't find a hole, however I did find a half moon chunk of piston right under the spark plug hole that looks like it has been erroded away. Could this missing volume be what is causing the lower compression. I found a bunch of rubber chunks in the coolant jacket, and I assume this is water pump impeller? Could this motor have been overheated due to a water pump failure and caused this piston to start going? I assume corrective action is to replace the piston and water pump impeller? or can I replace the impeller and run it for a while? Thanks for the input.

Bill

 

[Laddies]

Re: Low compression 1 cylinder 88 Mercury 60HP

Heat sticks rings, If it were mine I would repair the w/pump and test run before I did any major engine work. Mercury powertune works good as a pre lube before startup.

 

[SMOKNZ]

Re: Low compression 1 cylinder 88 Mercury 60HP

I just found out that the impeller was replaced by the dealer when we first started having this issue. I did find a few rubber chunks in the coolant passages near the bottom cylinder. Think this was enough to cause a big enough restriction in cooling water flow? The impeller replacement did not fix the problem with the melting aluminum being deposited on the spark plug shorting it to ground. I did read a few posts on a plastic washer or something that can melt down when the motor gets hot, does this pertain to this motor. If Teltale is good can I assume there is no issue with the coolant system?

 

[thefairlaneman]

Re: Low compression 1 cylinder 88 Mercury 60HP

Some things that cause a compression drop can only be seen by removing the head and checking the cylinder walls. A wore risk pin can cause side wear you cant see by looking into the spark plug hole.