Okay my truck seemed to get warm (210-216 degrees) if I was going slow pulling a load last fall. I ended up changing the water pump with a cummins genuine replacement because mine had a very small amount of play in it. I haven't ran the truck very much after that and haven't noticed it getting warm. The other day while sitting in the McDonalds drive thru for about 15 minutes the temp got up to 210 (thats where my Edge Juice/Att warms me of heat) and I thought no way can this happen.... It seems to me that the fan wont turn on as it should. If I turn on the A/C the fan starts and it cools down. I assume there is a coolant temp sender that controls the fan when A/C is NOT on and I want to replace this sender. I hope this will take care of problem as the fan does work with A/C on so I assume its not the fan. Could it be as simple as just needing a new thermostat? The truck cools down as soon as I start driving again and getting air thru the radiator. Also when I did the water pump I completely flushed out the cooling system and filled with coolant mixture I also got from Cummins(the red stuff). I am just wondering if any of you have been thru this before. Just a note... this did this before I changed my injectors also.

 

Try the HOAT type coolant for starter (Orange type coolant) helped me out when I had the same problem.