Engine coolant is what keeps your car engine from overheating. But by nature, it gets very hot and can evaporate through use. Thus, you will need to periodically check and top it off.
Do your homework. All cars build in the last 20 years or so have a transparent reservoir connected to the radiator. Originally this was meant to catch overflowing coolant, when the car overheated, and prevent it from running on the ground. Now it is used as a reserve for the radiator. This is a good and safe way to check and add coolant if needed.
Check the level of the fluid by noting the position of the fluid relative to the lines molded in the plastic. These are different for different cars, but you may have two lines “full” and “add”. Or you may just have one that says “maximum”. The optimal level should be just below the “full” or “maximum” line.
Lift off the lid of the reservoir and look in. You should see a bright green liquid. If you don’t see this liquid, add some from the bottle. You should add enough for the liquid to reach the line. If you spill any, wash it away with clear water.
Replace the lid on the the reservoir and you are done.
Pouring engine coolant directly into the radiator, is another method. But, this MUST be done when the engine is cold. The best time to check and/or add coolant is in the morning before you start the car.
Open the radiator cap by pushing down and turning to the right. Look inside, and make sure that the fluid is just below the fill neck. It should be bright green.
Pour the coolant in through the fill neck, until the level is just below the edge of the neck. Try not to spill any, as coolant can cause fan belts to slip if excess coolant is not removed.
Replace the radiator cap. Be sure it is tight. You are done.
