Do you use coolant or distilled water in a car radiator?

You can use almost any liquid in a radiator, but water is not the best choice. Water boils at 100 degrees (and your engine gets way hotter than that), also it freezes at zero degrees and in many places it gets colder than that.

Engine Coolent is an additive that you add to water or replace the water in a radiator. Coolant raises the boiling temperature and lowers the freezing temperature so that your radiator will work in the widest possible circumstances.