I was a guest on a boat powered by a couple merc 150s. The owner left the dock, revved up the engines to 3000 rpm and, after a short period, the starboard engine hot horn sounded. He slowed down. The telltale stream from this engine was weak in comparison to the port engine. The intake below was checked for obstructions. None found. He took off again and went up to 3000 rpm. Hot horn again. This time steam was coming from the hot engine. He's waiting for his mechanic to look at it. Does the appearance of steam from the exhaust mean that a head gasket was blown, or is it possible that the remaining cooling water in the head was just boiling hot? Has anyone else out there experienced this?
