Engine Exhaust/Yanmar Problem
First of all, let me tell you that I''m not a a engine exhaust/Yanmar expert. Just happen to be in a similar situation with a 60HP Volvo MD-22. What should I replace/test first? (
It is quite difficul to asses your point without looking the instalation.
However you have been fortunate for the time being.
1. Has yor Yanmar decompresors? If the answer is yes, until you are sure that you solve the water intake issue, your should start cranking your engine decompressed. After a couple of turns, move the handles down, and fire the motor. This procedure will make you sure that you have expelled any water, not only in the manifold, but in the cilinders at worse.
2. Check where the water was coming from, wether from the coolant or seawater. (Just in case) In the first case, you may just have a burned gasket, particularly if there is any water in the oil.
Now, if you have experienced troubles in starting (as if your batteries were down),I bet you check the compression just to figure out any harm in the gasket.
3. As you said, if it worked for 17 years, it is hardly to believe any fault in the design instalation. It is funny. I thought the same in my boat. But the previous owner seemed he shut the water intake valve before shutting of the motor every time since. By the time I bought the boat, he indidcated that, but stated that he did so, "to keep jelly and fisdh away!!" Fool of me: I believe him.
4. Finally, at least for me, there is a very good and complete article in Sail Magazine of september 1998, addresing the exhaust installation.