Re: 05 6.0 OVERHEATING WHEN TOWING BETWEEN 35-55MPH!
When you see some fan noise after a cold start (ie, the truck has sat overnight or equivalent), what you're seeing is called "morning sickness" and is pretty much standard equipment with any viscous fan drive. What happens is that while the truck sits still, some of the working fluid in the fan drive drains back into the working part of the drive, and so the drive is partially engaged when you first start the truck. Because everything is cold, and cold viscous silicone oil is, well, viscous, it takes a little while to pump it out.
The fan reacts to coolant temperature, air charge temperature, AC demand, and ATF temp. The coolant temp function is a ramp function, I'm pretty sure the air charge function is a ramp function, AC shouldn't be calling for fan at highway speeds, and neither should the ATF temp. So you may have a sensitive ear -- I can pick up one of these fans at about 1500-1600 fan RPM, but I'm always listening for it (one of the perks/curses of designing cooling systems for a living: you're always evaluating the performance of the system in anything you drive), but my wife can't hear it until about 2000 fan RPM (which would correspond to a fully-engaged fan at around 1600 engine RPM), but I doubt the fan is running fully engaged, particularly when running empty.
Of course, westbound I-70 across Kansas is one big gradual uphill grade several hundred miles long...
Former Cooling Guy
'99.5 F350 Lariat CC SRW 4x2, 7.3L 4R100
'99 Mustang Cobra, SCCA Solo2 E Street Prepared
'97 Dodge Neon, SCCA ITA/STU/EP Club Racing
Now I do subsea ROV tooling.