aluminum on spark plug

[Stlouisboating]

A mechanic was checking the compression on my boat's engine and found what he calls a "major mechanical failure". On a spark plug he removed from the engine he showed me aluminum which has soldered itself to the firiing end. He believes their is a hole starting in the #3 piston. I had another mechanic look at it and he verified this. Anyone else had problems like this? The engine is a 5.0 liter, 260HP, with a merc cruiser bravo 3 outdrive.

 

[Don S]

Re: aluminum on spark plug

Anyone else had problems like this?

Sure others have had that same problem. timing may have been way off.

 

[TilliamWe]

Re: aluminum on spark plug

Or you may have run the boat with a fouled (really dirty/scummy/grimy) bottom long enough that the increased drag caused the combustion chamber to get so hot you melted a piston.
Just for S and G's, try loosening the valves on that cylinder and re-checking the compression. if it increases, you "may only" have a valve problem. If it doesn't increase, you most likely have a piston problem.

Good luck.

 

[Don S]

Re: aluminum on spark plug

you "may only" have a valve problem.

Valves don't leave aluminum on the spark plugs.

 

[Bondo]

Re: aluminum on spark plug

Ayuh,...

For Whatever reason,+ there are Quite a Few,...
Your motor has been Detonating....
If the Detonation Issue hasn't gone Too far,...
Fixing the Issue might allow a reasonably long,+ Happy life for the motor...
But you've no doubt lost some engine life....

What were the Compression #s,..??

 

[TilliamWe]

Re: aluminum on spark plug

Valves don't leave aluminum on the spark plugs.

Duh, Don, I know that. But without seeing the metal in question (or testing it, for that matter) how do we really know it's aluminum? We don't. So you try one other thing to help you know how bad it can be. No harm in that, is there? I realize the heads need to come off, so you could just wait for that, to be certain... but I don't want to see the "mechanics" talk him into a new long block, without a tear down, and then find out he needed a $200 reconditioned Vortec Head.