Thermostat..... my truck all of a sudden was running in the 215-220 range when it clearly shouldn't have been..... and I have all new up to date stuff... ... like new radiator-hoses-water pump - thermal viscous fan drive/fan blade assembly... . I thought that if a thermostat had went bad --- (and this one only had 20,000 miles on and was only a year old) it would either stay open or shut... . Well check out the picture - notice the black like material that looks like a foam insulation you would spray around a window. It wasn't like this when I installed... . nor do the new ones have it. Changing to the new cummings thermostat (and this one was a factory one as well) solved the problem immediately! So - don't assume its not the stat - even if you think it shouldn't be... .