I just rebuild some things on my 86 sunchasser deck boat. It has the 4.3l mercruiser engine... I got the thing running ... new coil, plugs, wires, battery so now I replaced the impeller and some hoses along with a 140 degree thermostat. The engine starts good but once I get into gear and running it stalls with in minutes. My temp gauge is maxed out at 240 so my next idea is to replace the sensor?? Any ideas it’s engine S/N B403048
Measure the resistance of the temperature sensor when the engine is cold and when it's hot- if it remains the same, remove the wire from the sensor and make sure it returns to the lowest reading. If nothing changes, remove the wire from the sender at the gauge and see if it changes- if not, it's a bad or miswired gauge.
Are the exhaust manifolds hot? If not, and if you see a good amount of water coming out of the exhaust and at the transom, you may be OK from that standpoint. Did your impeller lose vanes? If it did, make sure to gather any loose pieces of rubber from the water intake slots. The boat I recently bought was badly overheated and I found the pieces behind the slots- they come out easily, by removing one screw from each side.
If the engine has overheated more than once, make sure the water shutter is intact, unlike mine. It was gone.
Have you cleaned the flame arrestor recently? That's the single most neglected piece I have seen on boats. Next time it stumbles, close the choke, to see if it dies or improves- if it improves, it's running lean. Have you checked the timing, lately? Is it at spec?
