Air trapped in the lines of your cooling system can make a perfectly healthy system run as though it's malfunctioning. These air pockets accumulate over time and can cause overheating by impeding the flow of coolant through the system. They also make the coolant level appear to be high when in fact it is just air pushing the coolant level up. Here is perhaps the best way to bleed air from your cooling system and make sure your radiator functions properly.
Step 1 - Mix Water and Antifreeze
Make a mixture of one part water and one part antifreeze. Pour it into the radiator, filing it up to the rim. Be sure to fill both the coolant or overflow reservoir with the same water and antifreeze mixture.
Step 2 - Turn On the Engine
Leave the radiator cap off, turn on your engine, and let it run until the radiator bleeds out air. It may take between 15 and 20 minutes for the engine to heat to the proper temperature and begin cycling coolant through. You'll observe the coolant level drop as the air is cycled out, and see air bubbles escaping from the radiator, and perhaps hear it gurgle.
Step 3 - Observe the Temperature
Observe the temperature gauge as this is happening. The gauge should decrease to normal or close to normal. This process helps the radiator feed the coolant more evenly, helping the system cool the engine like it's supposed to. It's no longer circulating air in your radiator instead of coolant.
Step 4 - Refill the Radiator
Completely refill the radiator and the coolant reservoir again. They will be as much as half empty since earlier it was simply air that made it seem full.
Step 5 - Replace the Radiator Cap
Seal the radiator with the cap. This will keep most of the air out of the system. Your car’s temperature gauge should be back to normal, and your engine should no longer have problems overheating unless there is a different issue causing it.
Step 6 - Other Methods
There are two other methods available for bleeding air from your cooling system. Some vehicles actually have bleeder valves specifically for this problem, located at the top or in front of the radiator. Open the valve and bleed the air trapped in the upper portion of the radiator.
You can also jack your car to bleed the air from the cooling system. This puts your radiator higher than the rest of the cooling system and helps force the air pockets closed. Keep the radiator cap loose to help the air move out during this process.