I have an 08 Boat with a Mag 350 MPI, Bravo 3. Boat is serviced regularly (Impeller changed annually). Last year it started doing something odd at the beginning of boating season. Randomly, it would get hot, the alarm would go off because it was getting hot. It never did it before. I had the boat serviced at the beginning of the season and the impeller was changed. I didn't think it was the impeller but I pulled it and checked the houseing. All was good. So, for good measue, I put a new tstat in and check all the hoses, no obstructions. Well, every now and then it would get hot, alarm would sound, I would get frustrated. One other thing. When the boat was serviced, they changed the exhaust manifold under warranty. Apprantly they were leaking.
So, this year I changed the impeller (the old one looked just fine, but changed it any way). First weekend I had it out it ran great. I took it out a second weekend and here comes the alarms again??? It went off twice. I let it cool off a bit and then drained the water from the engine at the lowest blue plug. It ran good after that and was flawless the next day. I didn't have my lazer therm gun with me to check the tstat, hoses, risers and block to see if it is really getting hot or just a bad sensor. I believe it is really getting hot. Oh, this happens all all speeds. Even at idle.
I have one friend with a 350 mag who hads similar problems. He said that a shop fixed it. According to the shop, the 350 has a problem with the bypass. Occasionally, too much water goes through the bypass and not enough to the engine to cool it. Apparently, they put a little restricter in at the bypass to force more water in the engine cooling hose.
Anyone else have this random overheating issue? Could something have happened when they replaced the exhaust gaskets? Does it need the restricter chingus fix? Am I getting some kind of air bubble in the system?