There are a few problems with using water instead of coolant in your radiator. The boiling point and freezing point of the water are not ideal for a coolant. In cold countries, the water can freeze up and block the radiator and the engine. This can cause damage to the expansion plugs and other small parts. Also, the boiling point of the water is much less than the operating temperature of the engine. This will cause the water to boil inside the radiator and causes pressure buildup. Finally, water is corrosive. A good quality coolant solves all these problems. So, you seriously do not want to use water instead of coolant in your car.