Coolant should be checked every 6,000 miles, or every other oil change. Check the level as well as the condition of the coolant. Also, note that coolant and antifreeze are two different terms that both refer to the same fluid.
Checking the Coolant Level in the Reservoir
Most vehicles have a translucent plastic reservoir (sometimes referred to as an overflow or expansion tank), which often has a black cap. It's very easy to check the fluid level on this kind of reservoir.
Have the engine off.
Open the hood and find the coolant reservoir.
Look through the reservoir to determine the coolant level. Some reservoirs have markings for cold and hot levels, while others may say MIN and MAX or ADD and FULL.
If the level is anywhere between the minimum and maximum marks, you don't need to add coolant. If it is below the minimum or ADD mark, add coolant but also have the vehicle checked for leaks.
A typical coolant reservoir. Note the MIN and MAX markings stamped in the plastic.
Checking the Coolant Level in the Radiator
While in most cases checking the fluid level in the reservoir is sufficient, it is a good idea to check the level in the radiator as well. Coolant expands when hot and contracts when cold, which is why it needs an expansion tank to go back and forth from. This movement is controlled by the radiator cap. If the radiator cap goes bad, it may let coolant into the reservoir but not back into the radiator, which can lead to a low coolant level in the system even though the reservoir is full.
Note: When checking the coolant level in the radiator, it is extremely important that the engine is cold. If you open a pressurized cooling system when the engine is hot, the coolant will vaporize and this can result in some very severe burns. Because this is so important, radiator caps will be labeled “never open hot”. Always heed this warning. To check the level in the radiator:
The engine must be off, and has to be cold.
Using a rag, remove the radiator cap by pushing down while slowly turning it counterclockwise.
Look into the radiator to determine the level.
The coolant level should be all the way up to the radiator cap. If it's full, put the radiator cap back on (push down and turn clockwise, making sure it seats fully). If it's low, be sure to add coolant but also have the vehicle checked to determine why the level is low. If your vehicle does not have a radiator cap, then it will have a pressurized reservoir. Do not open this reservoir cap while the engine is hot (the cap should have a warning on it).
What Happens if the Coolant Level is Too Low?
If the coolant level gets too low, the engine can overheat and you risk engine damage or blowing a head gasket. If the engine starts to overheat, shut it off and let it cool down. Do not open the radiator cap when the engine is hot, even if you suspect that it's empty. The same goes for a pressurized reservoir cap. Have the vehicle towed to a service center.
When checking the coolant level, also pay attention to its condition. It may be hard to see through the reservoir, so you may need to take the cap off to get a better look. If the cap has a warning that says not to open it when hot, be sure to wait until the engine is cold before opening it.
Coolant comes in a wide variety of colors, but all will be translucent. Green, orange, red, yellow, and blue are some of the colors that may be seen.
Coolant that is looks gunky, rusty, colorless, or has debris floating in it should be changed.
Coolant that has a “milky” appearance is the result of oil or transmission fluid leaking into the coolant. Have the vehicle serviced as soon as possible.
Like other automotive fluids, coolant deteriorates over time. Because of this it's generally a good idea to have a coolant flush done every two years. Also, your service center should check your coolant's freezing point when the vehicle is in for an oil change. The freezing point should be about -35°F or less.
Adding Coolant to the Reservoir
Coolant can be added to non-pressurized reservoirs at any time, but note that some vehicles will have a pressurized reservoir. Pressurized reservoirs are just like radiators in that they should never be opened when hot.
Non-pressurized reservoirs will have a cap that will just pop off (like the cap on a washer fluid reservoir does), while pressurized reservoirs will have a cap that screws on. If the pressurized style, the cap will also have a warning that states to not open the cap when hot. To add coolant to the reservoir:
Remove the reservoir cap (making sure the engine is cold if the reservoir is the pressurized kind).
Using a funnel, add the appropriate coolant until the level reaches the FULL mark. If the reservoir has markings for hot and cold, be sure to fill it to the appropriate level.
Put the cap back on.
Note that it's perfectly normal for the coolant level to fluctuate in the reservoir as it expands and contracts with temperature. Regardless of this, the coolant level shouldn't go below the ADD mark. If you have to add a lot of coolant, or have to add it frequently, have the vehicle checked by your service center
Adding Coolant to the Radiator
Just like when checking the coolant level, it's important to only add to the radiator when the engine is cold.
Remove the radiator cap (if you haven't already).
Using a funnel, add the appropriate coolant until it reaches the top of the radiator. Coolant is very bad for the environment, so be careful not to spill.
Put the cap back on, making sure it's seated correctly.
If the coolant level in the radiator was low and the reservoir wasn't empty, you should have the radiator cap checked for proper function.
Can I Use Water Instead of Coolant?
While you can add pure water to the cooling system in an emergency, you should have it flushed out and the proper coolant added as soon as possible. Of course, it's best to have the vehicle towed instead of trying to drive it if the coolant level is extremely low or a leak is severe.
What Type of Coolant Should I Use?
When adding coolant, you should use the same kind that is currently in your cooling system. This is often determined by color, but the proper type of coolant you should use for your vehicle can also be found in the owner's manual. For example, if your vehicle has green coolant in it then you should only add green coolant. If it has orange coolant, be sure to add orange, etc. The exception to this would be multi-use coolants which are designed to be compatible with any coolant. Sometimes referred to as “global” coolants, these can be added to any other color or type coolant.
Ratio of Coolant to Water
All types of coolant sold in stores come in two varieties: pure (or concentrated), or as a 50/50 mix of coolant and water (which is also known as premixed or prediluted). It's important to add roughly a 50/50 mix of coolant, which means if you buy pure coolant you'll need to mix it with an equal amount of distilled water before adding it to your vehicle. If you buy a premixed coolant, you can add it directly to your vehicle without adding water to it. The coolant/water ratio is important for a number of reasons:
If there is too much water or only water in the cooling system, the coolant will freeze at a higher temperature than it should (as high as 32°F). Also, water lacks the lubrication capabilities and rust inhibitors of coolant, which can lead to corrosion and increased wear of the water pump.
If there is too much coolant, the engine may run hotter and is more likely to overheat. While coolant protects better against freezing, water is actually better at cooling the engine than coolant is. A 50/50 mix of coolant and water also provides better freeze protection than pure coolant does.
Get Your Vehicle Serviced if You Notice:
A low coolant level, even if there are no visible leaks
Gunky, dirty, milky, or rusty-looking coolant
Milky looking oil (oil with coolant in it)
Problems with overheating
Coolant bubbling in the reservoir
White smoke coming from the exhaust (burning coolant)