How to Check and Add Radiator Fluid

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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, return it to the radiator, and dissipate it safely. Because of this, you need to maintain an adequate radiator fluid level, which means you need to check the coolant level and add to it when necessary.

Part 1 of 2: Checking the Radiator Fluid Level

  1. Park the car on a level surface.Ideally, you should do so after driving the car a short distance. You want to check the antifreeze or coolant level while the engine is cool or lukewarm, not hot or cold. If you have driven the car a longer distance, let the engine cool for several hours.
    • Do not leave the engine running when you check the radiator fluid level, and do not ever try to check the radiator fluid level when the engine is hot.
  2. Raise the hood.
  3. 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.
  4. 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.
    • Place your pointer finger and middle finger from one hand together and press down on the cap while simultaneously turning the cap off with your other hand. This will prevent a coolant burst in the event that the system is still pressurized.
  5. Check the radiator fluid level. The coolant level should be near the top. If there is any “Full” marking etched into the radiator metal, that is the level to where your coolant should be.
  6. Find the cap to your radiator overflow tank and remove it. Besides 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 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.
  7. 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 Protection Level.”
  8. 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 at a one to one ratio, or half antifreeze and half distilled water. 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.

Part 2 of 2: Checking the Coolant Protection Level

  1. Squeeze the hydrometer bulb. This forces air out of the hydrometer.
  2. Insert the hydrometer’s rubber tube into the coolant.
  3. Release the bulb. This draws coolant into the hydrometer, so it floats either the needle or the plastic balls inside the hydrometer.
  4. Remove the hydrometer from the coolant.
  5. Read the freezing or boiling point level on the hydrometer.If your hydrometer uses a needle, the needle should point to either a specific temperature or temperature range. If it uses a series of plastic balls, the number of balls that can float indicates how well your antifreeze protects the engine from freeze-up or boil over. If the level is insufficient, you will need either to add coolant or replace it.
    • You should test the coolant protection level in the spring and fall, and more often if you drive the car under extreme conditions.

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