I have a 1999 Mercury 150 HP carburetor type Salt Water Series. Engine was replaced at 70 hours by merc since the drive shaft rod cracked at flywheel. Since the day the new engine was installed in the old shell I have had sporadic overheat problems. Just got the engine back from the authorized merc repair shop to the tune of it costing me about $1000.00 for them to replace a stator and an outside switch box. These, I might add, were new in replacement engine 2 years ago which has only 60 hours on it. I also replaced two thermostats and impeller and pump myself. While doing the pump repair I pumped high power water through cooling pipes in case of any blockage. None found. Also had merc tech check pressure relief valve which he said appeared to be working properly. Still have overheat at 2800 rpm's which is 20mph for my boat. In six hours of use it has not overheated at higher or lower rpm's only at 2800-3000 .Warning buzzer comes on at this time and gauge reads about 220 degrees. Can't afford to keep playing this expensive process of elimination with the merc repair center. Anyone have any ideas for a fix? I've exhausted all mine.... Thanks
