I replaced my factory thermostat because it would require over 200 degrees before it would open and I feared it might stick shut. After it opened it would maintain 190 degrees. I replaced it with a Cummins thermostat #3954830 (190 degree) and it would run in the high 170's to 180. I thought it was defective so I replaced it with the same part, no change. The reason I want 190 degrees is that several years ago I tried a 180 degree and the mileage took a hit. Any suggestions would be appreciated.
John

 

Hmmm, I'm at 90,000 miles and
my water temp gauge seems to go to slightly over 200 deg on first warm up, then under to about 190, and cycles back and forth often. Is this normal or do I need to change my T-stat?

 

Once my factory thermostat dropped to 190 degrees it pretty much stayed there, except when towing then it was about 195.
John

 

mine does the same. has since new nothing wrong.

 

i have a 190 t -stat in my truck and i live in the north east which is cold right now,25 to 30 degrees during the day . truck struggles to get to and maintain 190 degrees while driving . truck usually hovers around 180 degrees.

 

rjm022-My temperature comes up when towing to about 190 plus. A cold day here (So. Cal.) is in the 50's. I guess these trucks are just cold blooded.
John

 

These trucks are designed to run and work hard. Fortunately, Dodge built them with an excellent cooling system. That means when they're not working hard and it's cold outside, the truck won't get up to operating temp.

I have a street/race car that's very similar. I built it with a huge alum radiator and a high performance pump. In the winter, I have to cover 1/2 the radiator so it can get up to operating temp. The engine itself is not cold blooded - a byproduct of power production (especially with gas engines) is heat. And this engine makes a lot of power.

In the winter, you should use some kind of front cover to block the cold air. You really don't want the engine to be running cold all the time.

Diesel engines run more efficiently at a higher temp. I can't find any real data to say what temp is best. But my own winter experiments tell me that a front cover in the winter gains a little mileage = efficiency.

If it were available, I'd install a 200* or 205* thermostat.

 

The thermostat is two stage. Partly open at around 180F and fully open at 205F.

.b0B is dead right about using a grill block (fall/spring) and a winter front (winter) for temp control.

Keep temps at/above 180F and internal engine friction is reduced. That means longer life & better fuel economy.

An ULTRAGAUGE (OBD reader) is a cheap way to monitor many truck parameters, engine temp among them.

 

I ran a 180 briefly and it butchered my fuel efficiency.

200 is no big deal AT ALL. Your description sounds like a properly operating stat.