Check and Refill Radiator Fluid

Many people are familiar with a radiator cap that you remove in order to add water to your car’s cooling system. A lot of cars still have a radiator cap on top of the radiator but the majority of cars now have a sealed radiator with an expansion tank elsewhere in the engine bay. The older type of radiator cap was a metal push and turn type whereas the modern type is a plastic screw type which is safer.

Cooling systems are designed to operate under pressure when hot and serious burns or scalding can occur if the pressure (radiator) cap is removed. Never work on the cooling system when hot. Always check the coolant level when the system is cold.

On the expansion bottle/ tank there is a max and a min mark and the correct level is more or less half way between the two. The main difference between the two systems is that the modern car has pressure in the expansion bottle/ tank while the radiator cap type does not and pressure is retained under the radiator cap.
If the level in either is above max you will have to remove some coolant and if under min you will have to add some coolant. As a matter of good safe practice always wear gloves, keep your face away and unscrew or remove the pressure cap.

The radiator cap type normally pushes down and turns first through 90 degrees and up, to release any pressure in the system. Then push down again and turn through another 90 degrees and remove.

Many people refer to checking the water in your car this is not an accurate statement. The coolant is a mixture of water, antifreeze and anti corrosion inhibitors. Normally you will purchase antifreeze from the auto store and mix this with water in a 50/50 concentration.

Mix enough coolant for your particular vehicle. Refill the system slowly through the radiator cap or expansion bottle/ tank cap.

You need to pour slowly to allow air in the system to escape. Once the level stops dropping you can stop pouring and start the engine without replacing the cap. The reason for doing this is because there will still be air trapped in the system and this won’t be expelled until the water pump is running which doesn’t happen until the engine is running. Once bubbles stop coming up through the coolant you can replace the pressure cap. If bubbles don’t stop it could indicate another problem such as a blown head gasket. Let the engine run after the pressure cap is replaced to allow the system to reach normal operating temperature which can be verified by looking at the temperature gauge on the dashboard.

Source: http://www.cararticle.net/how-to-check-and-refill-radiator-fluid-in-your-car/