Hi Gconlon -
What Flippen discibed is exactly correct! My guess is that the damage has been minimumal, because if there was allot of water you would not have started the engine and you would most definately blown something as you can not compress water or any liquid for that matter! So you would have cracked the head, blown a head gasket, bent a valve, or burned out the starter! Does your oil look like chochlete milk? If any of these are true you have a problem bigger than just replacing the manifold!
Yes if water, particularily salt water gets to the cylinders they will rust and cause poor seating of the rings! You can easily test for this with a dry compression check and or leak down test! All cylinders should be within 10% of each other from a PSI stand point, around 180 PSI +/- 10%. If you find one that is weak do a wet test, just pour a few ounces of engine oil in the failing cylinder and retest the compression! If the presure goes up you most definately have a ring issues. If it stays the same you most likely have a valve issue or a blown head gasket!
Now are you sure you have a cracked manifold? The reason I ask is that there is a surge valve in the exhaust system that is installed about 6 inches below the the riser! It job is to stableize wave action when the engines are off! Some times they stick or just don't close! When a boat is aft weighted and in a situation where wave crash on the stern the wave action can force water back up the exhaust past the surge valve into riser and once there into exhaust manifold finall ending in the cylinders! while the engine is running the exhaust pressure keeps the wave action at bay! This is one of the reasons that I hate seeing boats beached with the bow up and the transom down, drive up it takes little wave action to over ride the surge valve!
Hope this helps!
Mike -
