We are on the road from Rancho Jurupa Ca. to Moab Utah. The infamous Baker Grade west of Las Vegas brought us down to 15 mph 1st gear to keep engine temps under control. The ambient temp that day was 100+The rest of the trip has seen the engine run 190-200 degrees, but any increase in elevation brings us to 2nd gear to manage heat. 

The radiator "looked" clean to me before we started the trip. In Cedar City Utah we had a truck shop steam the radiator, and saw some evidence of dirt in the fins.