2001 Merc 6.2L MX reversion?
 

Howdy all. I'm working on my 2001 Dancer 310 cruiser with twin 6.2L engines w/ bravo III's. I noticed on port engine, the riser gasket was leaking externally; so I purchased new OE exhaust components and gaskets for the engine. When i pulled the first manifold/riser apart, it was apparent the gaskets were leaking both internally and externally. The other manifold/riser on the engine did not have an obvious gasket issue. However, traces of water was found at the bottom center of the manifold, and it had been infultrating the exhaust valves. No water in the oil however. I decided to pull the spark plugs, and the center plug on the side with no apparent issues was rusty. Not good. I did a compression check, and found that cylinder was 10% lower than the highest on that engine. I inspected the manifold and riser with a flashlight to check for cracks in the cooling jackets; non found. I don't believe there to be a head gasket issue, otherwise it wouldn't make compression. A leak down check indicates exhaust valve leakage, but this engine has not been ran since last October.

The stock set up on this has an extra 3" riser block before the elbow. Exhaust flappers were also intact; though i'm replacing with new regardless. I'm concerned i don't know where the water came from on that side that had no gasket failure. I'm wondering if this has been reverting a little water at idle. I'm not sure, this is my 2nd season with the boat.

The other engine, also has original exhaust, no visible signs of leaks. I decided to pull a center spark plug, and found it rusty with a drop of water on the end. I now ordered new exhaust to be installed on that engine.

Any further thoughts on this? I figured i need to start with replacing the 20yr old exhaust system and flappers before i can jump to a conclusion of reversion. I don't believe head gasket failure would be on both engines affecting same cylinders, with good compression numbers.

Thoughts?

Thanks for the help.

For freshwater boat I'd replace what's broken. My family has a freshwater boat with 6.2 built around same time as yours and I recall there was a premature failure of port side exhaust manifold at like 300-400 hour mark, but was limited to the one manifold. It was doing as you described, could see the external leak and the backside was all rusted up, just out of view.