Question

why radiator of an automobile is not completely filled with water ​

Answer 1

if there is a small leak in the system, when the coolant contracts, some air will be sucked into the system and less coolant from the overflow tank causing an air pocket to form at the top of the radiator.....

Answer 2

Answer:

If you mean antifreeze, antifreeze adds lubricants for the water pump, rust inhibitors, it cools better when the engine is hot, and lowers the freezing point so the coolant won't freeze in winter.