Merc.i/o 120h 4cyl. keeps overheating.removed thermostat,replaced impellar,water pump is clean & free spinning

Question

also replaced therm. housing as well as impellar housing 2X.Boat runs cool at the house with muffs,but when put in the ocean,runs hot immediately.Can someone please help!!! when the muffs are on I pulled the water inlet hose off the thermostat housing to check if the impellar was pumping water to motor,it was.I am completely baffled!!!HELP!!!

Answer 1

Mine did the same thing. It had pieces of old impellars stuck in the water jackets in the upper foot which were restricting the flow of water. It ran cool idling but would heat up quickly when accelerated. It was baffling because I could see water coming out of the foot, it just wasn't enough.