Tossing my 2 cents in with my flame suit on...

(better flame suit than Ebola suit any day)

I'm going to say possibly, just to CMA, a clogged radiator. Only because it was discovered many times while trying to diagnose overheating issues with customer's trucks.

 

I think the one question GaugePro is asking is how have you verified it's overheating? Does it boil over? Shoot the degas bottle with an IR thermometer? Digital thermometer in the coolant ?

I as well as anyone have all kinds of suggestions on how you should spend your money( cough radiator) but that is because I am taking it on faith that your truck is overheating....cause you said so. Imagine the feeling when you spend 100's to possibly 1000+( depends on if you do the work or not) on something you never verified was a problem, only to find out it wasn't broken, the idiot gauge was.

I fix X-rays for a living, I learned to "trust but verify" when a customer tells me something is not working like it used to or is supposed to. At least one time in ten they are describing " normal operation" and the only issue is I need to educate the customer, with much humility and tact. They are only people and memory plays strange tricks sometimes.

 

This has been at the back of my mind especially since they said the flushed the cooling system. It's a very broad term as you're unable to tell how they flushed it out.

Did they use water?

Or did they use Prestone? VC-9? Restore?

I/We are fully aware of that.....

You still haven't answered my question....

 

I recently had very similar symptoms on my 2002 7.3 with a manual transmission and about 158k miles.

After spending way too much money on diesel fuel trying to replicate the problem via blocking air path, etc., I should have started with the infrared thermo. My problem was the radiator.

I finally took the shroud completely off so I could do a true test (no fan interference) with the infrared thermo I found that there was a 17 degree variablilty in temp about 1/3 the way across the radiator. With the shroud off I could also see a small section where it had leaked so I replaced the radiator. After replacing the temp read a consistent 103 degrees across the middle of the radiator.

I also did a complete flush and replaced with ELC. I will say that the water flow seemed very quick and the flush did not really indicate it was blocked.

I took the plastic top off the old radiator and tried to push air from the compressor thru the "tubes" and approximately 67% of each row was blocked.

When trying to replicate the issue, it seemed like the fan clutch was only lightly cycling on and off. But it is hard to keep all air from getting to the fan without doing some type of Ohm test as indicated earlier in this thread. My mechanic suggested that I clean the coil on the fan clutch with the air compressor and it seemed to improve the performance--at least it seemed to cycle-on earlier than before.

I would say that there is no reason to doubt the "idiot" light because it gave an accurate posting of my issue--that is it said it was hot and it was hot!. Not having an in-dash gauge it was all I could rely on. Also the infrared thermo also validated the "idiot" light findings. Infrared also gave indication that the water pump was functioning. However, I am not sure if this method would work if the water pumper propeller was bad and not moving enough water.

This thread was very helpful in tracing my problem. Thanks to all who contributed.

Jim