The temperature of my CAT 9.3 is 97degrees C (205F) and my ISL Cummins was a little lower. The radiator's job is to get rid of the heat but allow the engine to run at its optimum temperature. Using a 15 lb radiator cap the big CAT uses two thermostats to make sure if one should stick shut the engine does not overheat. I am told by the mechanic that these thermostats tend to fail open which means the observer sees coolant temperature abnormally too low.
On a long hill climb the warmer coolant temperature is desired as it allows greater dissipation of heat. The radiator can pass a lot more BTUs when there is a greater difference between ambient temperature and radiator temperature. When a young man in the South we were wrongly advised to remove the thermostat so our old automobiles would run cooler. That was a mistake as the old leaking non pressurized systems and radiator could not pass enough heat to keep the coolant from boiling. With age and gaining of wisdom the thermostats were raised in temperature and pressures were increased to 13-15 lbs which allowed the engine to maintain coolant temperatures just below boiling but hot enough for the engine to run at greatest efficiency. Importantly this allows the radiator to pass greater heat amounts on the hottest days.
Just got back from the CAT dealer and sure enough one of my thermostats had failed and the engine and radiator were running too cool on a 100 degree day. After repair with new thermostats and the new red coolant with fancy inhibitors the engine got right up to design temperature and stayed in a rather tight range. Today's car companies take advantage of the modern designs by using higher pressure and temperature systems which TRANSLATES into a smaller and less costly radiator and higher MPG with tighter heat control.
Check on your exact engine but the non wet sleeve engines run a bit lower design temperatures like 185-190 and the wet sleeve models are very close to or just above 200. I don't know of any diesel that is designed to run as low as 160 F. That would make me suspicious of a bad thermostat. .
