My port engine is over heating as well. New impeller. Old one was great, new thermostat, new water pump. 5.0gxi 2003 280 signature. Doesn’t over heat idling. Only when rpms are increased on plane and off plane at about 2-2500 rpm as well. Does cooL down again but heats again when reving. Can the heat exchanger in the water heater be an issue? Starboard engine don’t have that connected to it. Also if I pull the thermostat out I can run it with it barely heating up. I ran it at about 35-4000 rpm for an hour around the lake and it didn’t feel hot at all and the gauge was just over 120.