Antifreeze has evolved along with heavy-duty diesel engine technology over the years, but you still need to dilute it with water at a 50/50 concentration. Simply topping off low coolant with straight water or pure antifreeze can degrade the freeze-point protection and lead to slushing, poor heat transfer, and engine damage.
For those of us in the A/C parts and service business, an improper mix also increases the risk of broken or leaky water valves. At low temperatures, liquid inside the valves will begin to ice up and damage the seals, causing them to leak.
Who cares, when it’s as cold as it’s been? You want water flowing through there and getting hot, right? Well, as temperatures warm up and you want that water valve to close, it might not turn off completely. If your drivers complain that they’re having trouble with their heater, you might need to add water valves to the work order—and remind folks about the need to maintain a 50/50 mix when they top off the coolant.