I've had my truck just about a year now and I've only seen it up to operating temp twice, normally it runs 160 to 165. During the winter months I made a cold weather front from a sheet of aluminum that blocked 95% of the radiator and it still only ran 160. Back in May I towed maybe 4500 pounds 750 miles or so round trip average speed was 70 to 75mph and it still only ran 160.  About 3 weeks ago I pulled the radiator and power washed all the oil and dirt out of it and installed a new 190 thermostat. I noticed it ran slightly warmer about 170 but its been back around 160 ever since. In less than a week I'm  taking a trip out west of the Rockies and would like to see it run hotter than 160 the whole way. I'm out of ideas as to why it wont run any warmer, I replaced all but the heater core hoses, new 190 thermostat and topped it off with all new green coolant mixed 50/50. Short of running a winter front all year anyone have any suggestions?

 

 

Gauge defunct? Sensor defunct.

 

Verify coolant temp with another gauge

Measure with another mechanical gauge or IR temperature gun.

FWIW I have dealt with the same problem for 8 or so years. It got so bad that when the temp dropped to 160 or less the Banks Ottomind tuner I had would quit working. I finally took the tuner out and the temps stabilized a bit around 170 or so. I even installed a cheap suncoast temp gauge. Now it seemed to read higher temps near 190 most times I looked at it, but the lead was short and the gauge was under the hood. That made comparisons iffy at best. it has since stopped working. I have done that same as you as far as new coolant and t stats, from a $15 Stant up to a $65 oe replacement and one cummins t stat with no change. I sent one of those t stats to Mike who ran it in his truck for some time with no issues. I have also installed 2 new temp sensors with no difference. my temp has since, over time, settled down to mostly around 180 to 184. I have no idea as to why. Even today I can be cruising down the interstate at 75 mph and the temp will, out of the blue, fall of to around 160, 165 and hang there a little while then go back up to 180 or so. This happens very rarely now. The only 2 ways I can get the temp any higher is to strap the 5th wheel on(15k #'s and 28'-8" tall) and hit the highway or put in the winter fronts and climb a steep grade when 60+ outside. That will get my temp up but with either as soon as I am cruising on level ground, it  is back to 180.

 I wish I could help you solve the problem but I have never figured out what mine is. Something in the wiring is my best guess. I do feel the engine is running at the proper temp but cant prove it. One day I am going to install another temp gauge that come into the cab and compare the two better. By the way I am reading my temps from my scan gauge that stays close to what the dash gauge is reading. Of course they are reading off the same sensor so they should.

I still think Dripley's problem is the chopped up electrical or damaged ECM on the temp sensor...

I dont think it is the ECM since the trouble began well before the oe ECM went out and the problem carried over to the new ECM. The low temp readings predate the wiring harness being cut also. I have since soldered the those wires back together, so i dont think it was the splices but still think it is in the wiring. I guess I cant rule anything out yet.

Is the temp sensor the one next to the thermostat housing?

Yes. It is fairly in expensive to replace.

I wouldn't run regular green antifreeze, get something for diesel. I'm running zerex g05 right now. Not saying that's the problem but it could cause other problems. 

Curious as to why you would not use the green ethylene glycol. I believe that is what came in mine and the FSM says to use it. Is there something I am missing?

I wouldn't run regular green antifreeze, get something for diesel. I'm running zerex g05 right now. Not saying that's the problem but it could cause other problems. 

I would also like to know why you say this. Thank you

I've switch over to yellow universal.

Do you think it is a better product than the green EC?

I use the red or orange stuff... Dex cool I think. 

I run the unverisal yello because its more common than the HOAT and typically cheaper. As for performance still have a perfect cooling system.

 

I would also like to know why you say this. Thank you

From hat I found out, even though our engine goes not have liner, there's still a chance to have cavitation. Better be safe than sorry. What I use now is Zerex GO5, and also got some Flletguard I might use in a future. 

I use the red or orange stuff... Dex cool I think. 

I would not use dexcoll it causes electrolysis, too many GM cars have bad intake and head gaskets failures and when taking them apart, aluminum gets real bad corrosion. I think the only reason they use that crap is for waranty it lasts longer but causes more damage. 

I would look at your fan clutch and thermostat. The fan clutch could be stuck in the locked position causing temps to stay down. Also the sensor needs to have a good source voltage to read accurately. Check the voltage to the sender and post what you find. I will try to find that spec in my Cummins manual and let you know what the voltage should be. 

Also reset your gauges. With the ignition off hold the odometer/trip meter post on the instrument cluster and while holding it down turn the ignition to on. The odometer/trip meter will do some funny code reading and recalibrate the gauge cluster. The codes that pop up will be associated with the gauge cluster in most cases. Let us know if this helps.

I have to say I have been thinking about the fan clutch on mine. I still hear it engage or disengage. It is hard to miss when fully engaged. Really roars. Maybe partially engaged it is not noticeable. At least to my worn out ears.