I've been having some problems with my truck running about 10 to 15 degrees hotter at highway speeds. Also when I accelerate past 70 or so my temp gauge will rise with my speedometer??? I took it in and the shop said I needed a new radiator, so they replaced it. Problem is it is still doing it! What could it be? Thanks

05 Cummins, 160k, just exhaust and CAI for right now

 

I would be replacing the thermostat, followed by the water pump...and I would inspect (read: give it a squeeze) the lower radiator hose to see if it still has the internal support spring/wire (to keep it from sucking shut).

I would also consider checking the sending unit for the gauge...at least verify the gauge is reading correctly.

 

Thanks so much for the help, also. Where is the sending unit and how do I check it?