I bought a truck recently, it had a bad tranny so i towed it home fixed the tranny and the first trip out it overheated. I pulled the t-stat and put in a new one and same thing. Ive tried 2 other t-stats since then and same problem. None of them have opened up. I ran it without one and of course it doesnt warm up to normal operating temp like it should but it doesnt overheat anymore and circulates just fine then. I even pulled the water pump just to inspect it and its just fine. All the t-stats ive used are parts store ones but i cant imagine having 3 bad t-stats. Anyone have any wierd trouble like this?

 

What type of overheat, a slow temperature rise towards the red, or the "Oh my god, the needle looks like a rocket heading to the sun" overheat?

If slow, I would pull the Radiator and make really sure the fins are clean. If fast, I would first make sure you had the stat in right, then I would get either a genuine Cummins or Stant PREMIUM from a Napa or other full line ( not discount ) parts store.

 

I've had real bad luck with auto parts generic stats, think they're made to be used with gassers. Even if they work they don't last long.
Only use Cummins p/n 3925466, never a problem.

 

I agree with patdaly and check out your radiator for restriction first. Feel across radiator does it have any cold spots? outside of radiator clean front and back? Clutch fan doesn't free wheel when spun?