Well been trying to track down my overheating problem. Temp is fine till get above 3400rpm then hits 180degrees while under load. Checked everything, replaced thermostat, checked risers, flush entire cooling systetm and nada.

Ran at the dock for a while this evening. Ran at 3600rpm for about ten minutes and wouldn't get over 150 deg. Ran at 4000 for 3-5 minutes and barely hit 155. SO seems that when engine is under load something is causing to work too hard and overheat. They are going to pull tomorrow so can check drive and fluid, etc. I did recently have to change to a aluminum prop when spun hub. Any other ideas as to what to look for?

You've not give any significant info.

Bravo or Alpha?
Stock or modified motors?
Thermostat temperature.
Automotive or true marine thermostat?
Hours on motors?
Salt or fresh?

Does motor run properly? What is WOT rpm?

And of course the motor runs hotter under a load. Why would you think it would not?

more info and we'll try to help.

Sorry was late when I posted and not thinking straight. Motor is a reman block installed two years ago Merc 7.4L Bravo I. About 200 hrs. Runs strong, only will run up to about 4200 with the aluminum prop. Is pushing a heavy 27 express cruiser about 8000lbs we (other boat is a Formula 223). Thermostat replaced is a merc 140. Won't get above 160 degrees if stay below 3400rpm while running, is only above that it hets up to about 180. Motor is RWC on the Chesapeake.