My 06 5.9l diesel run hot sometimes, I changed the Thermostst about 6 weeks ago, everything was fine until last night, it started running hot then drops off, It keepsd going back and forth. I flush the radiator today and change the thermostst again. Could the water pump be going out. . I have no sign of leakeage anywhere..
Thanks

 

My 03 did the same thing the fan clutch was out,put a new fan clutch in it was back to normal just remember to plug it in lol after we put mine in it was still running hot so we took it in to a mechinac he fixed it in 30 seconds lol.Was just a simple mistake

 

I have an 06 5.9 did the same thing it was the fan clutch. It would only do it when I was pulling my 5th wheel slow when there was not enough air moving across the radiator.

 

Turn on your a/c and cover the condensor with a blanket and verify that the fan is actually engaging....if it doesn't, problem solved!