Finding water in the cylinders.... I had a similar problem and was told that the silent choice Y pipes caused a vacuum even when just utilizing at idle speed. This vacuum caused water to be sucked down into the cylinders by the compression (I'm not a mechanic, so forgive me on the laymen terminology). Merc (off record) told me that the 500 EFI's and up shouldn't be operated w/ any form of Y pipe. I added CMI's, diverting the water from the pipes, and haven't had any further problems.

Merc warrantied everything but the labor. I was 1 month out of warranty.

I have 500 EFI's, but another guy I know w/ 575's had the same problem. Hope this helps!

I'd like to look at your plugs real close. They're going to be the answer to whether it was octane related detonation or fuel starvation. Look for teeny particles of shiny aluminum on the insulator.

I don't think I see any chicken tracks on the pistons...

My 575's don't have Corsa type exhaust, just mufflers on the tips. The head gaskets blew through to a coolant port and allowed water into the pistons.

I looked at the plugs and didn't notice anything shiny on them and the pistons arn't pitted. I will look closer at them tonight.

Thanks
Roger

Good luck Roger. I hope you figure it out.

Mark