You have a clogged radiator. The fan and clutch are the primary driver of temperatures at idle as the radiator does not need to flow much coolant at idle speeds. At highway speeds the radiator is the primary driver of temperatures. There is plenty of airflow through the radiator at highway speeds even without a fan but the engine needs a lot of coolant flow because of the load and the rpm. Your symptoms are classic plugged radiator. Because your truck idles at normal tempWIth the inline you can remove the thermostat element completely to test the system but I think you will see the same temperatures.

 

Great discussion on cooling issues and experiences which we all have encountered. Extinct's feedback extremely valuable given all of the work that he has done in this area.

 

ok i flushed the cooling system three times with distilled water and ready to add back coolant and the thermostat

so just to be sure - should the hole in the thermostat be up or down in the in-line connector?

and should it be on the front grill side or the engine side of the reusable gasket?

 

Flushing won’t necessarily clean a plugged radiator. If it’s plugged with sludge (maybe your case?) you will likely need a solvent that will dissolve the sludge. If it’s calcified (not likely in your case as the rad is fairly new and you probably didn’t use tap water in it) I doubt that you can clean it. Also if it’s plugged with radiator stop leak (also not likely your case) you probably can’t clean it.

 

i used a radiator cleaner to help with sludge

and then I used distilled water to flush it 3 times until the water was totally clear

didn't help at all evidently since I have the same issue at highway speeds

 

Yeah, the tubes are so small in the Disco radiator flushing is of no use. I have had customers with brand new radiators bad out of the box. If you have removed the element and it does not overheat idling but does on the highway then the radiator is plugged. Just need a new one.

 

ok

any suggestions on a new radiator? OEM? Another Nessens? Alisport?

 

I recently put a Nissens in my P38. The reviews are good and they’re not expensive. I got it from Rock Auto.

 

No need for OEM, Niessens is fine.

 

thanks

ordered a replacement from Rock Auto this morning