Cat c12 engine temp

Have you verified the gauge is accurate? Fan stuck on?

 

"I am assuming the gauge is good." You may be right but it is best to verify. Tap into the computer and see what temp the ECM is reading.
Water pump - is the coolant circulating?
Temp sending unit

 

"Water pump is 2 mths old." Not relevant. What brand part was installed? A lot of cheap junk out there. While we're on the subject of parts what brand thermostats were installed? Maybe the pump impeller is not turning which could cause the issue you are having. See if you can tell if the coolant is circulating. Or you may have to remove the fan and fan clutch (at least the belt(s)) and turn the water pump by hand to see if there is any resistance.

"if the water wasn’t circulating wouldn’t the coolant be overheating" Not necessarily. When cruising there is a lot more airflow and with the cold air ("it’s been in the 30’s") at cruising speed (think wind chill and the air is even colder) the temp gauge would drop like you are seeing. If the temp gauge is accurate which needs to be verified.
Don't forget the Thumbs up/Thank icon.

 

That's good. How long have you owned the truck? As in have you had it for years and it never did this or this is the first winter you've had it?
What temp did at run at in the summer when the air temp was much higher and not in the 30's? Did the problem start after the water pump was replaced or other work done on the truck?

According to a post on The Truckers Report forum, the Caterpillar C12 engine should run between 190°F and 210°F and be controlled by the fan cycling. Depending on the fan in temperature, it could go as high as 225°F1.

So I'm wondering if it has too much radiator capacity (cooling more than the engine and truck were designed for). Wrong thermostat opening temp. Even though the fan is not stuck on as you say, if the fan is coming on too early/too much due to a faulty switch would cause what problem you are having.

I would verify that the thermostat(s) are not stuck open and the correct one is installed.
The ECM controls the fan so need to get into the system to see what the ECM is reading for coolant temp.....

 

I know you said new thermostats. Did you fit new thermostat seals in the housing? Any corrosion or pitting in the housings.

Either you have a thermostat opening too early or water is somehow circulating when they are closed.