If it is stuck open, then the coolant is flowing freely. It's not being held in the radiator to cool, so it overheats when you're sitting still.

When my truck was overheating the temp would climb while cruising (anything over 50mph). The temp would also go up if I ran the a/c at idle. My radiator was clogged.

I'm just saying t-stat because it's a cheap fix. I'd change the t-stat and rad. cap, about $25 in parts, and see where you're at. But my money's on the radiator. Check those cores for cool spots. The radiator in my truck was a single core pos.

Let the truck idle and listen for the clutch fan to cycle. You should hear it "roaring" when it engages.

If I had it to do over I'd change the radiator at the 1st sign of any type of overheating. Overheating is what blows the head gaskets. A 3 core radiator is the 1st mod I'd consider. If you run the heater too much, to compensate for overheating, you're going to blow the heater core (ask me how I know smile ) and you don't want to go there. I think I'd rather pull the heads.