One of my dock neighbors is pulling his hair out. He has a mid-80's vintage Hunter 25.5 with a Yanmar 1GM diesel. He put the boat in weekend before last and when I met him at his slip to help with that first docking I could hear an alarm as he was coming down the fairway (it was about a 4 minute trip over from the lift well). He figured that it was an impeller issue (raw water cooled), although he does not use the boat a lot, he also doesn't remember when he changed the impeller last. He let the boat sit for an hour or so, started it up and let it run for 15 minutes, no alarm. Fast forward a week, last weekend I helped him change out the impeller, the old one looked fine btw, no missing fins. He started the boat up and within a minute the alarm was going off. I verified with him that it wasn't oil pressure, he said the alarm was a water alarm on his panel. I checked the exhaust flow and although it was difficult to see from the dock, it appeared that he was getting water, at least some water, out of the exhaust(it exits under the boat, not out the transom). The motor itself seemed to be running cool, didn't smell like we were burning up the exhaust hose or anything.

Other than a bad sensor, is there anything else we should check? I'm not a 1GM expert by any means, but he knows less about the little diesel then I do.