With the exception of air-cooled engines, radiators are the main cooling component of a car, and there would be no way to prevent a car from overheating without a radiator, according to 2carpros.com. In most situations, you should use a mixture of 50 per cent water and 50 per cent coolant in your radiator. If you use pure water, it could freeze in cold temperatures and crack the engine block, or the water could boil in hotter temperatures and cause problems, according to autoupkeep.com. However, you can use pure water in an emergency to get you to a place to buy coolant.
Turn the car off and wait for the engine to cool.
Lift the bonnet of the car and locate the cap for the coolant reservoir.
Remove the cap.
Pour the water into the reservoir.
Replace the cap and close the bonnet.
Avoid pouring cold water into a hot radiator.