It will Overheat at high RPM's and when towing a trailer. Ever thing looks good. The Clutch fan is new. Any ideas?

 

replaced anything lately besides the fan?
possibly upside down thermostat or incorrect install of viscous fan, etc.

 

The transmission was serviced, and the radiator was replaced about a year ago. Other than that nothing has been done to her. Ever thing is exactly the way the factory put it in.

 

does your fan cut out at high rpm?

 

Try replacing the radiator cap if everything else checkes out ok.

 

Could it still be the cap even if the radiator is not loosing fluids?

 

Are both the upper and lower rad hoses hot? Maybe there is enough flow to allow for cooling until there is a high demand on the engine.

 

If the radiator cap doesn't help and all else looks good then I would change the thermostat with a newly tested one. As Weed has indicated, restricted flow.

 

Yeah, for sure swap out the thermostat. If it isn't opening completely, flow will be reduced and you'll get less than full cooling. If it isn't opening at all or it jammed, you will have a highly-restricted flow of coolant.

Clean the radiator if you haven't already. Flush from the fan side outward towards the front of the truck. That'll get all the bugs and goo out of there.

Make sure you have enough coolant in the system in the first place - check the level in the overflow tank, but also check the level in the radiator (don't open hot) to make sure that it's filled also. The radiator cap should allow overpressure to spill into the overflow tank, but should also allow the system to draw coolant back into the system when things cool down.

 

Sure hope so. The fan isn't needed above about 30 mph. Centrifugal fan clutches went out a long time ago.