Stumped on a Yamaha Overheating Problem

I have a problem with a 1996 yamaha 30HP (oil injected). It spent its first 10 years working in saltwater, and since then, it's been only in freshwater. About two years ago, it developed an overheating problem for the first time. It runs fine below half throttle, but above half throttle, it overheats after running a while (1-5 minutes). With warm water and WOT, it takes about one minute for the alarm to come on and the RPMs to be limited automatically. The whole time, the tell tale stream seems strong, and the water coming out gets warm, but not hot.

I've tried everything in the manual, I can't figure this out:

1. The water pump (whole pump, not just impeller) has been replaced twice. Pressure at the themostat seems good.

2. The Temp sensor has been replaced. When it overheats, an IR thermometer near the top spark plug reads about 190 F, which is the temperature that the alarm should be sounding (per manual). The lower two cylinders are reading about 20 degrees cooler. So it seems to really be overheating (no a false alarm or oil issue), and it seems to be the top cylinder primarily.

3. The thermostat has been replaced. Also, I ran it once with no thermostat, and it behaved the same.

4. There is no poppet valve on this model (per the manual), so I can't blame that.

5. I rebuilt the power head over the winter. The cooling passages under the exhaust cover and head cover were a little discolored and rough, but there were no big deposits or blockages that I could see. I replaced one slightly-scratched piston and all the rings and gaskets. After the rebuild, it ran exactly the same as before (still overheating).

So now I'm stumped. Any ideas? If I pull the head cover again, is there something more subtle I should be looking for? Maybe a blockage somewhere else?

Thanks!

Re: Stumped on a Yamaha Overheating Problem

when were all these parts replaced.

Water indicator comes off before water runs through the motor so it will not be hot.

something is not allowing enough water through motor to cool properly, are you sure water tube is not cracked, or the water tube grommets are in good shape.
I have seen the one on power head swell and restrict water flow into power head
How about the dampener near water pump, corrosion could allow exhaust gasses into water pump

Re: Stumped on a Yamaha Overheating Problem

Thanks for the input. The parts were replaced piecemeal over the past two years, but in each case, the problem remained unchanged even immediately after each part was replaced.

Today I pulled the water tube and checked the grommet and damper seal. I hadn't closely checked them previously, so I was hoping it would do the trick. They all looked good, and when I test-ran the motor after reassembly, the overheating problem was still there - in fact perhaps even worse (overheating even at low throttle now).

I'm still looking for possibilities, if anyone has any other ideas.

The cooling water seems to be circulating ok. Could the power head be generating more heat than the cooling system is designed to remove? Mixture too lean? Bad oil delivery? Something in the powerhead causing friction? Is there a good way to check how much water is making it through the engine?