As I am traveling down the road with no load on the truck, as I hit a small rise for maybe a 1/2 mile my temp gauge starts going up a little and then the fan comes on and it will turn off once I get over the rise. With a load, even on a flat stretch the fan will come on and the temp go back down and the fan will turn off and in a short while will repeat itself. The truck has 215k on it. the radiator fins are not plugged. I can see on the temp gauge that the needle is starting to move up. The coolant it where it should be. EGTs are very good. Am I looking at a new radiator and should I replace the water pump at the same time.

any recommendations on radiators or pumps would be good

Your thoughts are appreciated

Larry

 

Possibly a clogging radiator, that would be my first choice to try. But ....the water pump is SOOO easy to change, not terribly expensive....I'd do them both while you flush the system.

I would also buy a cooling system flush product to run in it for a few days before i do the radiator and pump.

 

Basically this sounds normal to me.
Maybe your T-stat is sticking a wee bit and jerks open, letting a gush of hot water into the radiator, which in turn turns on the fan.
Had this on a 97 this summer, new Cummins t-stat fixed it. 3 aftermarket t-stats did not- always temp oscillations under load. It seems that you can fix the aftermarket t-stat by drilling a 1mm hole into the plate so that water is always passing through, this seems to make them react quicker.
But I prefer the original part, it simply works.

While you're at it take a look if you have rust or scale buildup in your engine and radiator. If yes, flush it with a good cleaning set, also available at a Cummins dealer, and fill with the correct antifreeze.

HTH