My truck has about 72,000 miles on it and just started overheating today. The temp is ok when running down the road but as soon as I slow down or stop the temp starts to rise.
It has not boiled over yet but it has pegged the temp gauge and I can feel the coolant boiling in the upper rad hose.
I was thinking that the clutch fan was bad but when I give it a spin by hand it doesn't even turn a full turn before stopping. I thought that if it was bad that it would continue to spin.
So I am leaning towards the thermostat being bad but if that was bad wouldn't it run hot going down the road?
I did notice running down the road that occasionally the temp gauge would creep up a little above where it normally runs for a few minutes and then would come back down to normal.

I was planning to start packing parts for the Nats in Columbus tomorrow and this will definitely screw up those plans.

Thoughts anyone?

electric cooling fan not working.motor?? relay?? most likely the motor.

Thanks for the response but my truck doesn't have an electric fan for the main radiator, just the A/C condenser.
It has a belt driven clutch fan.

Agree it sounds like a fan problem. A new T stat wouldn't hurt. I guess if the T stat was only opening partly, it may be enough for the highway. I'd go ahead and put one in. I'll add its worth triple checking coolant level in the radiator after its all cooled off first.

As a diagnostic, can you watch what the fan does while hot? Run it and let it idle until the guage climbs then feel for air flow in front of the radator. Pop the hood and see what the fan is doing also.

FWIW I changed the clutch on my 2006 Durango at about 130k. Apparantly the factory installed the clutch with locktight. I had to make a holding tool to get enough leverage to remove it. I also tried a "torkflow" brand clutch from AZ. No good, it howled like an air boat in winter. The replacement from MOPAR worked fine.