Your car's radiator is the heart of its cooling system, which also includes a fan, water pump, thermostat, hoses, belts and sensors. It directs coolant around cylinder heads and valves to absorb their heat and return it to the radiator and dissipate it safely. Because of this, you need to maintain an adequate radiator fluid level at all times, which means you need to check the coolant level and add to it when necessary. Here's how. Checking the Radiator Fluid Level
1. Look for the radiator cap. The radiator cap is a pressurized cap near the top of the radiator. Newer cars label the cap; if yours isn't so labeled, check your owner's manual to find it.
2. Wrap a rag over the cap and remove it. The radiator and overflow caps absorb engine heat from the coolant; using a rag protects you from being scalded.
3. Check the level of the radiator fluid. The coolant level should be near the top. If there is any sort of "Full" marking etched into the radiator metal, that is the level to where your coolant should be.
4. Find the cap to your radiator overflow tank and remove it. In addition to the radiator tank, most modern cars have an overflow tank for the radiator fluid to expand into when it gets hot. You should normally find very little fluid in here, if any. If your coolant level is low in the radiator and nearly full in the overflow tank long after the car has been driven, take the car in for servicing at once.
5. Check the freezing and boiling point of your coolant. Over time, the ability of your radiator fluid to absorb and dissipate heat declines. You test the freezing and boiling points with an antifreeze hydrometer. See the directions under "Checking the Coolant's Protection Level."
6. Add coolant as needed. Add the fluid to the overflow tank if your car has one; otherwise, add it to the radiator. (You may want to use a funnel to prevent spillage.) Under most driving conditions, antifreeze should be mixed with distilled water on a 50-50 basis. In more severe climates, you can go as high as 70 percent antifreeze to 30 percent water, but no higher.
Do not add fluid while the engine is still warm.