Truck Repair: N14 CUMMINS CELECT PLUS - HIGH COOLANT TEMP PROBLEMS, n14 cummins, coolant temp

Question
MY 1997 CENTURY CLASS COOLANT IS OVERHEATING.TOOK TRUCK TO DEALERSHIP,PUT IT ON DYNO THEY COULDN'T FIND ANYTHING,TECH STATED DYNO CEL CAME ON AND IT WAS READING 251/04 BUT COULD'T FIND CODE IN MANUAL.TOOK TRUCK OUT TO PICK UP LOAD WITH EMPTY TRAILER.COOLANT SYSTEM WORKS FINE,FAN CUTS ON AT 205 F,BUT UNDER LOADED TRAILER 40,000 LBS TEMP GOES UP TO 230 WARNING LIGHT COMES ON.

Answer
First thing to check is water pump belt for tightness. Then radiator for being plugged from bugs(not likely).Century class trucks have good air flow across radiator.205F sounds low,should be closer to 210-215F.But if warning light comes on its 220F atleast.Then change thermostat & check water pump pressure at the block.Should be a fitting at the rear on exhaust side.25PSI at high idle is good.