Basically, if it doesn't boil up, you don't have a (serious) problem.
If the cooling system really is duff, it will BOIL when worked hard, especially at low road speeds, like low range, big trailer, steep hill.
The optimum temperature for a diesel engine to run at is VERY HOT, the hotter the better in fact. The fact we run them at about 90 degrees C is simply because we use liquid water as the coolant medium. Some racing aero engines were cooled by deliberately boiling off the water, so the engine ran at well over 100 degrees C. This is not a good system, as steam pockets form, the areas round those go red hot smartish, and at 400 mph about 1,000 feet from the ground that is a bad plan.
People with air-cooled diesels such as Magirus Deutz would be horrified to find out how hot their cylinder barrels get under load. SIZZLING hot. Even the V W Beetle engines got way hotter than 100 degrees C.
If it ain't boiling up, don't fix too much.
CharlesY
