Sounds like it could be a combination of problems. The thermostat is easy enough to replace and cheap enough to buy that you should definitely replace it. I think an expensive one would still only be about $7 at AutoZone.
And run the truck in the driveway or garage while keeping an eye on the electric fan. In my experience, the electric cooling fan can be faulty--it's supposed to cycle on and off as needed while the engine is running, and stay on steadily while your A/C is on.
But if you turn on the A/C and it doesn't come on, or if it shuts off while the A/C is running and it won't come back on, get a new electric cooling fan. If it's shutting off when it's not supposed to, you'll find that tapping the shroud with your hand will kick it back on.
(Had a similar issue with my girlfriend's '98 XJ. We got stuck in Savannah, GA for FOUR DAYS trying to get back to FL from NY--while Pep Boys replaced EVERYTHING ELSE in the cooling system. I kept telling them to check the fan, and they said it was working fine. $500 later, they had replaced the water pump, which was new from the start. They had also swapped out the good thermostat I'd put in for a crappy one. And they flushed the system. It wasn't until we got back to Tampa that I said, "Screw it, I'll get a cooling fan." Got one for $80 from the junkyard. We no longer have the vehicle, but for the following year there were no problems.)
Tuesday, July 29th, 2008 AT 6:44 AM