this winter it didnt matter how long my truck was running the temp guage wouldnt move. i put a new thermostat in it last week and now when sit at a idle the guage moves up, the longer in runs the hotter it gets. today i sat for about half hr and the guage was reading about 200* but as soon as i take off it goes back down to about 150* within about 2 miles then slowly goes up a lil bit. is this normal? where should i start looking to get it right?

 

the cummins motors are known for having a decent fluctuation in there temp readings. what kind of thermostat did you put in it?

 

May want to change your sending unit it may be going bad. The fluctuation between 190 and 150 is normal though.

 

also might be a bad fan clutch