something that has not been mentioned, The FAN CLUTCH. yeah they can break also (well not break) but fail to lock the fan up.

 

I did test it and it works. Locks right on time with temp.

 

Well, you have eliminated all but 2 things

1. The radiator
2. The radiator cap

I thought fan clutch too, but if it can cool while not moving (as stated), it is unlikely to be your problem.

 

Radiator cap is good. The radiator looks like it has flow from what I can see.

I have updated the owner and I have told him thats its up to him as to our
next move. I dont know what to think at this point. If I do take this motor apart
or replace parts I would like to find the smoking gun at this point.

I spoke with a Dodge dealer tech today and he is doing the same thing we
are. Its a crap shoot at this point. I guess I need to make a move and hope
for the best.

Thanks for all the help guys and girls. I will let you know what happens.

Peace:

 

I had this happen once,on a new truck.92' d-150 with a 360 in it. turned out to be the radiator.My block was not properly cleaned out before assembly,filled the bottom 1/3 with casting sand. Radiator flowed like crazy at the top where there was no obstruction,but no flow at the bottom.It was odd,it cooled at idle well enough to keep the temp in the normal range,but going down the road it would over heat.Went to the local Rad shop to have the rad checked out and the tech called me an hour later to come look at all the sand in the cores!

 

Horseapples. Your the second one to say that. I had a tech from the Dodge store tell me
the same story as to casting flash left in the block and how it broke away from
the bottom of the motor and collected in the radiator.

We are going to deside today as to what to replace or remove next.

This has been a fun one.

Thanks for the help.

 

I'm also in with the radiator possibly being clogged. I had a 2000 Dakota where the radiator was inadvertently contaminated during a flush and change. It got flow, just not enough.

In addition to a flow check, they performed a temperature check before and after the radiator (and in other parts of the cooling system such as the reservoir) and were able to peg it to the radiator.

 

ask the owner if he's been towing this load since he bought the truck and it just started overheating?

Could the tranny be running too hot and the in-tank cooler is helping to keep the temps up there?
is there an oversized tranny cooler? I'd consider bypassing the in-tank tranny cooler and just go with the biggest tranny cooler available. Have a look at the tranny fluid and make sure it's not burnt.

also find out if the owner ever used tap water in his radiator.
have a look at this thread... https://dodgeforum.com/m_410483/tm.htm

 

I've been kinda hanging back watching this discussion. There is one more thing I can think of that I haven't seen mentioned as yet. The bottom hose collapses under higher rpm load. I'm making a small assumption here, that if the gentleman is towing, he is most likely using a lower gear, there by increasing his rpm and flow demand at the pump. Get the engine up to full operating temp, and continue to increase the rpm's to near the full range. At no time should you see any collapsing in the hoses to and from the radiator.
I was confronted with this exact problem years ago with a Big Block ford, drove me crazy, and cost me the price of a new radiator that I didn't actually need, as well as all the other assorted stuff, as has already been replaced on your friends truck.

 

and, the lower hose has a spring in it to prevent it from collapsing... yank it and look at the condition of the spring, it could be corroded and broke inside. you may want to replace the lower hose anyway if it's old.