Mercruiser V8

[kenJ]

A work mate was relating his latest boat problems. Over the winter rather than winterize he decided to leave a heater in the engine compartment on his Sea Ray, twin I/Os don't know the size. Every couple of weeks he went to the boat and idled it for a half hour or so. Got ready to use it this spring and it had a bad miss in the stbd engine. After a bunch of trouble shooting they discovered water in cylinders 4 & 6. Mechanic said it was a common problem with MC, at idle they tend to suck water into the exhaust rather than expell it. Sounds like hogwash to me, has anybody else heard of this?

[CDK]

I've had the same problem many years ago, but only after the seawater had eaten a hole in the thin manifold wall between exhaust and cooling circuit.

If the manifold and the valve train are O/K I doubt this could happen. With stuck valves in cylinder 4 and 6 some water could be pulled in.
In a very extreme case waves may cause a backwash that reaches cylinders with their exhaust valves open, but your friend probably does not leave his boat for the winter in a place where that could happen.

[gonzo]

I would bet it is a crack in one or more of the castings

[thudpucker]

There's one more way you can get water in the cylinders that is innocent.
If you run it in the driveway with the muffs on, and the outdrive way up high, you can put water back up into the exhaust manifold from the hose.
So after you shut the engines off, you get out of the boat to shut the water off and water from the hose has run back up the leg, through the riser and into whatever exhaust valves that are open.

I'm fairly sure thats why they tell you never to run the hose on hi pressure while the muffs are on.
Never run the engine on the muffs with the outdrive up.