Consistent overheating problem 135hp Merc Optimax Saltwater (2004)

Last year I had the local Merc mechanic do the 100 hr service on the engine. According to the Smartcraft digital gauge, the motor seemed to run on the hot side, so the mechanic supposedly r/r the thermostat too - now this year its acting flakey - it starts fine, and at idle speeds, it will warm up to about 140's to 150s F temp (thats in hot summer weather now with hot bay water too) - I dont think that is abnormal - Its even better if I gun it and run at cruising speed planed off, or even near WOT speed - yesterday it ran about 110-120 degrees F at speed & full plane - so that seems fine - the quirk on this darn engine right now is in-between speeds/rpms - if you slowly accelerate from idle speed up onto a slow plane, where you have the stern down low in water, and bow is up higher out of water - say about from 12mph to 18-22mph - Well if I go up on throttle slowly, and stay on that in-between speed (Not up on plane), the dog gone temp goes wild - it keeps jumping up from say that 140-150 idle water temp, right up quickly into the 160s, 170s, then it shuts itself down (programmed to by smartcraft software I assume) and says "Reduce Engine Speed" and it slows the engine back down to idle automatically - obviously to keep it from overheating and damaging the engine - I am no expert at all on these Merc V6 Optimax Saltwaters (2 stroke) - can any mechanics or mechanically knowledgeable people out there help me figure this out - I just cannot understand why it keeps overheating so rapidly at the mid-range speed/rpms. I can only assume it may have something to do with the angle/pitch of the outboard motor when you are sailing at that say 20 or 30 degree angle - maybe there is a dead spot or circulation problem created around the cooling intakes at that angle ?? (totally guessing here) Any help or ideas or someone who's had this happen to them is Greatly appreciated - PS just out of curiosity, what is the max engine temp these motors should reach before damage would occur - if the smartcraft digital gauge I have is accurate, it seems to shut the engine down to idle at somewhere in the 170's or 180 degrees F temp. Thanks for your help.

Re: Consistent overheating problem 135hp Merc Optimax Saltwater (2004)

When you run the boat at the point just before planing speed, you place a lot of load on it. Load means heat. You could also have a weak waterpump impeller, and you could have water pressure just below the poppit valve threshold.

To paraphrase a doctor's resonse. BoatCollector: "My outboard overheats when I lug it onto plane". Mechanic: "don't do that!"

If the waterpump impeller has not been changed since the motor was new, it is time. Other than that, if you must run at that speed, get a lower pitch prop.