I have an 02 mercruiser mag 350 mpi. A few weeks ago, I had a full service done - impeller included. I took the boat on the water last weekend and ran all day no problems. Yesterday, I launched and by the time I came back from parking the truck the boat was overheating. I shut the engine off, pulled it out of the water, and inspected the water pick up for any blockage. Thinking it was clogged somewhere (the water was full of small plant debris), I relaunched it and it overheated again. Frustrated, I took it to the shop who hooked it up to the hose. Boat ran great, no overheating. The shop thought it was an air pocket (I started it out of the water a couple times for a few seconds).
I took it back on the water immediately after - overheated again! Why would it overheat on the water but not on the hose?? The shop thought the impeller was fried, so I took it home and pulled it off. Impeller looked perfect, no blockage at the inlet or outlet.
My thoughts are that the boat sucked something up. I'm considering sticking a shop vac on the hose where I can. How do you go about unclogging the line? My second thought is a bad thermostat, but wouldn't it overheat on the hose?