gary wrote:

Take thermostat out and try it.

Do you mean I should try to run the D4D without the thermostat or take it out and test the thermostat?

Take thermostat  out and run tractor 
 

Well, I didn't remove the thermostat, BUT I put it to work, giving it a good workout with land clearing and digging. There were no issues at all—no overheating, nothing. Two hours later, still no problems. Temp was just slightly above the opening temperature of the thermostat.

When I drove the tractor back to the shelter (~400 yards) I used the second gear. I believe the coolant got hotter. It wasn't overheating, but the gauge still indicated that the coolant temperature was rising. I let it sit idling and checked the actual temperature with an infrared thermometer: 190°F, so still okay. Within 5 minutes or so, it got back to 180°F.

Pleeeease correct me if I'm wrong, but to me, it looks like the radiator and thermostat are fine, but the "gear" (the torque converter?) generates quite a lot of heat in the second gear. Does this make sense?
What could cause this?

Check your messages
Bob

The last new Cat thermostat out of a Cat box I saw said " made in China" on it.

Is the outside of the radiator really clean? Get a second person to hold a light on the fan side and look from the other . You need it so 90% or better of fins and tubes are clean, no pockets of caked dust. I ran a friends Direct Drive D4D on light road grading and it looked good but over heated easily. He took it home and spend hours cleaning the outside, made a world of difference. Not familiar with the torch converter drive to know how much that can be improved. But I have made great improvement by rodding the inside of the tubes.

I have also seen US name brand temperature gauges fail at very few years, so double check things with the hand held one.

Not a D4D thing or even a Cat thing, but I had the same type of problem on my PU after I changed the water pump and thermostat.

I changed it about this time of year and ran fine right at the same operating temperature for months. But one Mid June or July afternoon the truck said it was about 102 outside and it started to overheat with the AC on. It was fine on a mid 90’s day, but once the outside temperature went over 100 watch out. I got all kinds of advice. If you ran a heat gun on it really didn’t show that the thermostat was stuck, but I started with the thermostat and as soon as I pulled it out it something was out of place. The thermostat could only open partway or it was wedged crooked don’t recall exactly.

Kinda wished I had taken a picture of it or still had it but that’s another story. Somewhat quick and cheapest fix and never had the problem again.

you can get a dud one. remember they dont inspect every part. that would cost too much. and definitely clean the fins. bent and broken fins dont help either. depending how it is might be worth having it rebuilt. ask your local rad shop.

Did you check the temperatures of the inlet and outlet of the trans oil cooler? The trans cooler could be partially plugged.