Engine temp gauge showing low temp?
 

Usually runs right at 190* at hwy driving but in past week have noticed that it indicates 140-150* after 30 minutes of hwy running. No problems, no leaks, fluids all fine. Have 255K on the engine...I'm thinking maybe thermostat stuck open?? Any suggestions? Doc

Change the thermostat with a good quality replacement and see from there. In my opinion its one of the cheapest things to just so and see if it works. If your really determined to not spend unnecessary money you can pull the thermostat and test it with water if its opening prematurely. Put a pot of water on the stove and measure temperature with the thermostat in the water And watch the thermostat as it heats up. If its stuck open when You pull it its obvious. Way I see it if something is as cheap as a thermostat then just throw the part at it. Its not like throwing 100s of dollars in guessing which extensive testing would make sense.

I agree just changed my thermostat last week had the same problem wouldn't leave 140. New thermostat and it's fixed cost me $23 at O'Rielly

If it is a stock thermostat get rid of it. Dodge had a bad bach and installed them anyway, same thing happened to me.

I've never had issues with cummins thermostats. If you run a stant you MUST drill it or it WILL overheat.

I've had issues where it takes longer to warm up running unloaded in low ambient temps (>30F). The '07 didn't fully warm up for a half hour coming back down the mountain from skiing a couple weeks ago.

Winter fronts solve that problem.

Well I should have said mine was a 99. genos talks about the thermostate. Good luck.

whats a stant? Are they designed to remain open on failure? How is that accomplished? Doc