My 2001 Ram Cummins with 240k miles tends to run a little warm when pulling a small trailer. It has pulled the same load for most of these miles but recently I have noticed the temp gauge needle shows it running little warmer than I remember in the past. It does not overheat and the temp seems to fluctuate between normal and hot. I have installed a new thermostat and there has been no change. Any ideas?

 

fan clutch?

 

Check your radiator for gunk.