- Find the coolant reservoir. Follow the hose that comes from the top of the radiator cap–it will lead to the coolant reservoir. It is usually a clear plastic tank with full and low indicators on the outside.
- Open the cap and add coolant to the coolant reservoir, using a funnel to avoid spilling. Coolant is a mixture of half antifreeze and half water. In a pinch it is OK to add only water or only antifreeze. (On some cars, it is difficult to see the full line on the coolant reservoir. Bounce the car up and down while looking to see how full the reservoir is.)
- Put the cap back on the coolant reservoir.
- Add coolant to the radiator as well if the reservoir was completely empty. Make sure the engine is completely cool before opening the radiator cap.
- Some older cars don’t have a coolant reservoir; in that case, add the coolant directly to the radiator.