I was up in the mountains yesterday and all of the sudden my temp gauge flew way up. I shut the truck off, popped the hood. Figured I had a bad thermostat. Left it there overnight, and took a new stat up and put it in. I fired it up, but the fan didn't budge. It wouldn't even move with the engine off by hand. It stayed cool long enough to make it home, but I'm worried about the fan. What would cause this? The metal cylinder between the engine and the fan spins, but the fan doesn't... Any help is appreciated.

 

Sure sounds as though there is a issue in the fan clutch assembly somehow.

 

that's odd...usually the fan clutch fails and engages fully. But if it were to fail the other way you'd still get some free wheeling just from the rotational force of the fluid in the fan clutch..

If it's not spinning at all then I would check the fan shroud, and make sure the fan isn't being stopped by something.