So, you take your boat out for the first time after a long winter, only to find it starts running poorly, and you have a lot of milky oil on the dipstick and itís way over full.
What happened? Chances are very high that you have a cracked block.
Yes, people like to think the head gasket is bad, but the fact is, they very seldom go bad in a way that lets water into the engine oil.
Second hope is that the exhaust manifolds are leaking. Again, probably not. When manifolds or risers go bad, they leak water into the cylinders when the engine is not running. You then get a hydro lock when you try to start it. If it leaks while the engine is running, the exhaust blows it out the back of the boat and you wouldnít notice it.
Even if the water did lay on top of the piston, if the rings are good enough to give you compression, the water is not going to leak past them, at least not very fast, and you probably wouldnít see any accumulation of water in the oil, only a slight milky color if any change at all.
How can you check the block? well, the best way is to drain the water out of the block, block off all the hoses going to the exhaust manifold/s and anywhere else, and see if you can pressurize the block with air. It should hold 15 lbs. If it doesnít and you canít pump it up, then you have a problem. Sometimes you may be able to hear where the leak is but probably not able to pinpoint it. If it doesnít hold air pressure, then itís time to start looking. On V6 and V8 engines, pull the intake manifold off. You should see the cracks just above the lifters. On the IL4 or 6 engines, pull the lifter cover and have a look.
Letís say you found the cracks. Now your next question is going to be ďHow can I fix it?Ē
The answer is simple, you canít. Itís time for a replacement block or engine.
BUT I DID Winterize !!!!!!!!!
Sorry, but if you have a cracked block, it either wasnít winterized, or it wasnít done correctly.
Here is some reading on the procedures and how not to do it. Items 6 and 7 are what you are most interested in right now.
Here is a service bulletin from Mercruiser that explains where to look for water intrusion.
So now you have a little more backround on what, why, and how to, do some checking, then come back and start a new thread of your very own, with what you found or didnít find and Iím sure you will get plenty of help.