Cummins m11 overheating radiator been flushed and all ok but still runs hot under load but cools quickly at idle

Hello and welcome to Just Answer! I would be more than happy to assist you today. If you have no check engine lights for coolant temp verify the gauge is working properly through the Cummins insite program, ensure the fan operates at 205 degrees, you need you use a temp gun and and verify that the thermostat is not sticking and that that the water pump is circulating water properly. Allow the engine to warm up. Check the temperature of the upper radiator hose. If the hose is not hot and the engine begins to overheat, the thermostat is probably stuck closed,
Allow the engine to warm up. If the engine is running hot, feel the upper radiator hose. Try to squeeze it. If it is hot and very hard, the engine has probably overheated at some point. A hard hose usually indicates a blown head gasket that is allowing pressurized gas from the combustion chamber to enter the cooling system A repair shop can do a test for the presence of gases in the cooling system