We found that bypassing the thermal switch on radiator to make fan run continuously when key is on, ended the overheating.....of course thats after we replaced the head gasket, then the warped burnt head and gasket again.. in beginning on hot day, it would overheat in 1-2 miles.... We tested their settings for switch. The fan engaged at 238 degrees if I remember correctly .. and the switch is on the COLD side!!! how hot was the engine ??? 250* -260*?? The heat in the cab was unbearable, I would be drenched in sweat in minutes. .. now with a new adjustable switch and engine staying 150*-160* , the cab is pleasant.. I would also Check yours to see if head gasket is fried. look for air bubbles in recovery bottle while idling hot. and / or water in oil.