I think my fan clutch is bad. after market  or dealer oem  what do you suggest ?  if after market which one ?

I have read about aftermarket clutches not being as smooth or variable on their engagement, so they seem  like the run more. They also don't seem to last as long. 

Personally I would stick with an OEM fan clutch. 

 

Why do you think it's bad?

fan runs all the  time and overheating under load.   I will buy oem  fan clutch no after market

Over heating with the fan running would worry me. Maybe something is clogged and not allowing either good water or air flow? I know the 2nd gens are prone to the radiator clogging from the crank case breather. But dont know enough about the 4th gens to offer any advice.

Overheating with the fan running is odd, maybe the fan isn't fully engaged and stuck where it's at. 

this is a 4th gen 6.7 I have talk to mechanic and was told the it may run because it has the oil in it . but I never hear of feel it fully engage when the temp is up. and my fuel mileage has dropped 2mpgs which is another system of a bad clutch. the radiator is clean and was flushed last summer. this has led  me to the clutch  

Have you scanned for codes? The fan will set codes but they usually don’t illuminate the CEL. 

yes  I have scanned found no codes 

Fan's that run too much or too little should throw a code, so go over your symptoms some more. 

 

How do you know it's running all the time? Does it vary rpm?

 

Describe the overheating.