"If the boat is from about 199
"If the boat is from about 1997-1999, they had a service bulletin about this issue and I listed all of Mercruiser's suggested steps in another thread. If you call a Mercruiser dealer, ask for a copy of bulletin 97-17 dated July 1999. They probably won't know anything about it because most dealers don't read the bulletins and at this point, I doubt if many techs or service managers will go back and read old bulletins. For that matter, when I called Mercruiser for a copy of it, the new guy in the technical department didn't know about it, either. I asked how long he had worked there and when he said it was a few years, I told him to ask one of the old timers about it.
Running rich will definitely cause this, along with running it hard and shutting it off without letting it cool down. Engine compartment not being ventilated well will contribute, too, since the compartment temperature will be higher than what the motor likes."