- 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.
- Never open a radiator cap on a hot engine. The radiator is pressurized and the coolant could spurt out and scald you.
- Some German cars have pressurized coolant reservoirs.
- Wait for the engine to cool down before opening the cap to add coolant.
- Do not leave coolant out where children or pets can get to it. It has a sweet smell but is very toxic.
- A small hole in a radiator hose or a loose clamp could end up costing hundreds or even thousands of dollars if the coolant leaks out and the engine overheats.