I know this has been covered before, but I just wanted to add my two cents. I had a problem with the engine overheating when I ran it above about 1500 RPM. I decided it was time for a complete cooling system overhaul anyway so that is what I did. New fresh and seawater pumps, hoses, clamps, heat exchanger, thermostat, full flush and new antifreeze. This was on a 1985 M25 engine on the C36.
Although I was extremely careful in trying to avoid any trapped air in the system, I still ended up with some. I would run the engine without a thermostat and after a few minutes it would begin to run a little too hot. I would try the air bleeding valve on top of the thermostat housing, got no air out. I would shut down the engine and check the coolant level and there would be no change.
Finally I remembered reading somewhere about running the engine revs up to wide open momentarily as the engine temp begins to climb into the too hot range. It said you may need to do this several times. Sure enough after several tries I finally got any air pumped through and was able to add coolant. I could then run the engine and it remained down at about 130 to 140 degrees. I put the new thermostat back in and the engine runs at a consistent 160 through all RPM ranges.
So if the air bleeder does not work for you try revving the hell out of the engine.
C36 "At Last"
1985 hull# 369