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How to Check or Refill Engine Coolant so Your Engine Runs at an Optimal Temperature

Photo by United States Marine Corps. CC0 1.0 Public Domain.

Keeping your car’s engine at its optimum temperature ensures that you get the most out of your engine. Most of the time, however, a cold engine isn’t a problem; your engine will naturally heat itself up while it’s running. However, if your engine coolant isn’t maintained at the correct level, your engine can run too hot or even overheat.

What is Engine Coolant?

Photo by Peretz Partensky on Flickr / CC BY-SA 2.0.
Photo by Peretz Partensky on Flickr / CC BY-SA 2.0.

Engine coolant is a water-based liquid that serves many purposes in your car’s engine. Its primary purpose is to transfer heat from the engine to the radiator. In the process, the hot coolant exchanges heat with the outside air, dumping the heat outside of the engine before recirculating back into the engine.

Although coolant is based on water, it contains several additives that improve its ability to keep the engine running smoothly. These additives include lubricants to make the coolant flow through the engine better, anticorrosion ingredients to protect the metal of the engine and other ingredients to both raise the boiling point and lower the freezing point of the coolant. Engine coolant is also sometimes called antifreeze because it helps to prevent liquid from freezing in the engine.

Multiple Types of Engine Coolants

Photo by dno1967 on Wikimedia Commons / CC BY-SA 2.0.
Photo by dno1967 on Wikimedia Commons / CC BY-SA 2.0.

Although most of us lump all engine coolants under the same term, there are different types of coolants. The easiest way to tell the difference is by color; all of the different types of coolants have their own unique colors. Older vehicles typically use green coolant, which is based on earlier technology called inorganic acid technology and lasts for about 36,000 miles. Orange coolant is based on organic acid technology and is more commonly seen in new vehicles; this coolant will last about 150,000 miles before it needs to be replaced.

The different types of coolants can be mixed in small quantities, but they perform less effectively when mixed. If your coolant is low and you need to top it off, you can refill your coolant tank with any type of coolant or with water; topping off with water is fine, but the extra additives in engine coolant are important for long coolant life. In addition to the generic orange and green coolants, most automakers have their own brand of engine coolant. Use this specialized brand in your engine whenever possible; it’s been formulated to work optimally in the conditions typical for your vehicle.

How to Check Engine Coolant and Top Off if Necessary 

You should inspect your engine regularly to make sure you have enough coolant in your engine. When you check engine coolant levels, make sure that your engine has been off for several hours; keep your engine turned off for the duration of your check.

Locate the coolant recovery tank; it will be a translucent plastic tank and should be located near the top of your engine. Open the cap on the tank and inspect the liquid inside. The coolant level should be between the “Min” and “Max” fill lines. If you notice an oily residue or floating sludge in the liquid, or if the coolant is colorless or rust-colored, you most likely will need to flush your coolant system.

To top off or refill engine coolant, use a 50/50 mixture of antifreeze and water and fill up to the “Max” line in your tank. Avoid adding only water to your coolant except in emergency situations; antifreeze has special anticorrosion properties that will protect your engine. Without these properties, your engine may wear out prematurely.

What to Do If Your Engine Overheats

Photo by Soupeurfaive on Wikimedia Commons / CC BY-SA 3.0.
Photo by Soupeurfaive on Wikimedia Commons / CC BY-SA 3.0.

Failing to keep enough coolant in your engine can cause it to overheat while you drive. If your engine begins to overheat, pull off to the side of the road and let your engine cool down; this should take about 15 to 20 minutes. If you can safely access your hood, you can open it to help your engine cool down faster. Don’t remove the cap from your radiator while your engine is hot, however. Removing the cap from a hot engine can cause hot coolant to spray out or damage the engine.

After the engine has cooled down, you can attempt to drive again. If the engine continues to overheat, try to get to the nearest service station so that you can deal with the issue safely. Don’t be afraid to call for a tow if you don’t feel comfortable driving to a service station; driving with an overheated engine can cause permanent damage to your car.

Adding Water to Coolant: How and Why?

Pure antifreeze is actually less effective than a mixture of water and antifreeze. For optimum performance, you’ll want to dilute pure antifreeze with water. Mixing antifreeze with water will alter your coolant’s freezing and boiling points; the exact temperatures will change depending on the ratio of water to antifreeze. Consult the antifreeze bottle when mixing with water; for most conditions, a 50/50 mix of water and antifreeze is best, but there may be certain circumstances that demand a different ratio.

Servicing Your Coolant System: How to Refill Engine Coolant after Flushing

From time to time, you may need to flush out your coolant and replace it with new coolant; the exact timeline will depend on the type of antifreeze you use. Before flushing your coolant, make sure that your engine is cold and turned off. For extra safety, keep your car on a level surface and engage the parking brake.

Locate the drain valve for your coolant and place a container underneath it. Ensure that your container can be fitted with a tight-fitting lid before beginning. Open up the drain valve and allow the old coolant to flow into the container. Set this container aside, cover it tightly and label the container clearly.

Antifreeze is toxic, but it has a sweet taste that may attract the attention of children or pets.

Using a flush and fill kit, run water through the system to flush out any accumulated sludge or debris. After the system has been thoroughly flushed, close the drain valve. Mix up your replacement coolant according to the instructions on the bottle, and then refill your cooling system with the newly mixed coolant. After replacing the cap, your car will be safe to drive again. Check the coolant levels after you’ve driven it for a few days; top off your coolant if it’s lower than it should be.

Engine Coolant Safety

The image is illustrative and the cat wasn't harmed in any way. Photo by Robert Couse-Baker on Flickr / CC BY 2.0.
The image is illustrative and the cat wasn’t harmed in any way. Photo by Robert Couse-Baker on Flickr / CC BY 2.0.

The combination of antifreeze’s sweet taste and high toxicity makes it dangerous to leave lying around. Always store antifreeze in tightly sealed containers that are clearly labeled. Keep these containers stored up high and out of the reach of children or pets who might be tempted by its sweet taste; lock the containers away if you can.

If you’re storing antifreeze, keep an eye out for the signs of antifreeze poisoning.

Recognizing the symptoms of poisoning early may help save someone’s life. Symptoms may include:

    • Rapid breathing or lack of breathing
    • Blurred vision or blindness
    • Blood in urine
    • Leg cramps
    • Rapid heartbeat
    • Low blood pressure
    • Convulsions, dizziness or headaches
    • Fatigue, slurred speech or lack of alertness
    • Blue lips and fingernails
    • Nausea and vomiting

If you notice any of these symptoms, seek immediate medical care. Antifreeze poisoning can be fatal or cause permanent damage, so time is of the essence.


Spent antifreeze can and should be recycled to prevent environmental damage and eliminate risk to children and pets. Most auto parts stores will take your used antifreeze for recycling.

Engine coolant is vital to the health of your engine. A little bit of vigilance can prevent an inconvenient breakdown or a costly repair bill. Make sure to check and refill engine coolant regulary.

Do you check engine coolant levels regualry? Did you ever have to refill engine coolant?