I am running a 6.5 turbo diesel that overheats under load only when it is sitting the cooling system functions properly. Any ideas?

 

Clogged up radiator maybe?

I;m also gonna move this to the 6.5 forum so it will get a little more exposer

 

what kind of truck??? cause if its a early model 6.5 you may need to move up to the newer setup......i would also try cleaning the radiator really good, maybe go have it dunked in a chemical bath.......

 

Many "hot 6.5s" are fixed with thorough rad and ac core external cleaning, to do correctly is about a 3 hr minimum process, it's amazing how much crud accumulates in the cores fins, use lots of low pressure water to flush the debris, high pressure wash will fold over the fins and make the problem worse.

What kind of stats do you have in it, have you checked in pan of hot near boiling water to see if opening fully, check pot water temp with IR gun or candy thermometer to see if opening at the stats open set point. Is the fan clutch working correctly.

between the 2 cores is a collection point for junk, best way to clean only way that really worked for me was to remove the rad, spray commercial ac core cleaner into both cores (watch your painted area when doing the ac core if leaving in the truck) spray it & let foam, continue doing this until no dirt comes out, minimum 3 hrs to do it correctly, if you are not going to to that it's a "band-aid" effort.

Start with that and you'll be amazed at the difference, as a daily driver that should suffice for you, if you do any towing with any weight at all there are some better flowing pumps and retuned fan clutches that help with bigger than average towing needs, how big do you wanna tow depends on the size of the wallet, my rig is capable of sustained highway speed towing of 18K# of trailer & backhoe.