How to Check a Vehicle's Coolant/Antifreeze The radiator in your vehicle cools your engine and needs water and coolant (antifreeze) to function. Keep the following points in mind as you check the level of the liquid in your cooling system and add more,if necessary: Rather than open the cap on the radiator,just check to see whether the liquid reaches the"Full"line on the side of the coolant reservoir shown here. It's part of the coolant recovery system. If the liquid doesn't reach the"Full"line,open the bottle and add a 50/50 mix of water and coolant until it does. Some coolants are premixed,so check the bottle to see whether you need to add water or just use it as-is. Only in an emergency should you add only water to the coolant system. Most modern engines have aluminum cylinder heads,which require the protective anticorrosive properties of antifreeze. A 50/50 mix of liquid or coolant is usually sufficient ... Read more »