I did a test and made a bracket that locked my fan clutch, ran real cool, that's why I thought that it was airflow related and replaced the clutch.

 

Is it possible that your new fan clutch was bad? Now mine is locking and unlocking randomly, so I have to replace it soon, but I can really tell when i am driving. Trying to keep a steady 55 or 60 on short shallow hill, it can overheat, but with that fan clutch locked, I can do whatever I want problem free. It never seem to recover from getting hot after a hill, so it only gets that much hotter on the next one.

 

I feel the need to add this to the mix. I own a 92 Ford F350 with a 7.3 Diesel that sports an aftermarket ATX turbo system. Bought it 3 years ago and have had no issues pulling my travel trailer or just driving down the highway - until recently. I find that as long as I drive in town (30 - 45 mph). the temp stays in the normal range (between the R and M of N O R M A L) as soon as I get to highway speeds (60 - 70 mph) the engine O'temp light comes on about 10 miles down the road. This is with an empty truck, no trailer and no A/C, outside temp at 60 to 75 degrees. Once it reaches O'temp there is no cooling down without pulling to the side and shutting off the engine. I even tried turning on the heater and going max fan speed and heat in the cab to try and help. It only delayed the inevitable red light by 3 - 5 miles of driving.

The shop has found and replaced a bad clutch fan (you are right on the OUCH part) and I have also replaced the radiator cap since the old one was not allowing the antifreeze to empty into the overflow tank as the engine heated up.

As I was in the process of entering this information the shop called and found the radiator was clogged and informed me that it would be $400+ to Rod it or $800+ to replace it with a Ford OEM radiator (Double ouch!) I opted for the lesser of the two evils since the truck only has 100K miles on it. BTW they are replacing the Thermostat and both hoses while it's apart. Will let you know if this fixes my problem.

Signed - over a grand in debt so far

 

OK first thing put on a new water pump. I replaced mine and 2 impelers were boken off and layying in the bottom. chemicaly flush the system(this will clean any crud off the cyl. wall that makes the coolant move faster). make sure you add coolant conditioner back. I bought a trans cooler and a coolant filter from carquest and hooked them in the return line from the heater core. this adds disipating area too the cooling system.(trans cooler in front of the rad. like normal) next replace your oil filter with a new power stroke type. psd's use the same oil cooler so the oil filter mounting base is the same. (6.9-7.3idi filter is 51742 wix about 6" long - 7.3 PSD filter 51734 wix about 9" long) That extra length will cool the oil better cause it stays in the filter longer, oil cools the engine too. also will clean your oil better especialy if you get the 51734EX virsion. to rule out a rad. it should cool the water 10 degrees from top hose to bottom hose. all those little mods will add up.