Here's what my thermostat seems to be doing. The temp gauge needle parks itself halfway between 140 and 190 degrees, and just sits there for hours on end. This was on a 1,600 mile, 3 day trip. As you can see I'm driving 65 with the AC on. So this makes me think the thermostat is stuck open. Anybody agree?

 

If that's different behavior than it usually shows, (I assume it usually cycles constantly back and forth like most of them do) I think you're right. I would throw a new stat into it if it was my truck.

 

Yes, the gauge used to register a high of 190 and then dive down as the T-stat opened and cooler radiator water came into the engine.

Now then, where can I get an ORIGINAL Cummins T-stat? A 190 T-stat that does cycle? Does anyone know the Cummins part number for that T-stat. I'm not interested in the newer ones that maintain a constant temp.

Edit: What I left out is that this T-stat is new--I just had it installed a month ago. It worked fine for two weeks and then this happened. So a follow up question is can I use the front and back seals that the new T-stat has, or will I have to buy new seals?

 

Don't know if they still make the 190, but the factory 180 is Cummins # 3928499, spendy at 40-50 bucks, but it is what it is if you want factory operation.

94-98 12 Valve Cummins Thermostat with 2 Sealing O-Rings ( you might find it cheaper elsewhere,m this was just the first one I found for sale )

And yea, I wouldn't worry about re-using the seals, I have used them much older than that over.

 

Thanks Pat.