What Is A Coolant Reservoir For?

Woman Looking At Her Car Engine

A problem with your car’s coolant reservoir can lead to an automotive disaster with your car stalling after overheating or worse, your engine block cracking due to the excess heat. Problems with your coolant reservoir can also lead to poor gas mileage. To prevent this from happening, it is important to understand the role this reservoir plays in keeping your car running cool and smooth.

This article will detail the purpose of the coolant reservoir and how it works to keep your car’s engine from overheating. It will also explain several of the most common symptoms of a faulty coolant reservoir and potential fixes for those issues. With a little knowledge and some preventative repair, you should be able to prevent the problem from becoming an automotive disaster.

What Is the Coolant Reservoir For?

Cars need a coolant reservoir because their engines produce excess heat in their operation and without a reservoir, this heat would expand the metal parts of the engine causing them to warp or even crack. This is true of traditional internal combustion engines, electric cars, and hybrid electric/internal combustion vehicles.

With internal combustion engines, the gasoline is mixed with air in the combustion chamber where the spark plug provides a spark that combusts the mixture, moving the pistons to run the engine. This creates a great deal of waste heat that must be dissipated. Electric engines are powered by batteries and as they supply energy, these batteries heat up and must be cooled.

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How Does It Work?

The coolant reservoir supplies the coolant system with the coolant that it uses to keep the car’s engine from overheating. The coolant system also includes the radiator and fan, thermostat, water pump and the heater core of the car. These parts work together to keep the coolant flowing through the system and to cool down the coolant before pushing it back through the engine and the heater core.

This type of cooling is called indirect liquid cooling. As the coolant moves through the engine it absorbs some of the extra heat and this heats the coolant. The hot coolant is pumped into the radiator where it moves through the small flat tubes while the air from the fan cools it before it is pumped through the engine again.

Modern cars are designed to keep the engine at its ideal operating temperature. The potential damage to the engine from overheating has already been discussed but an engine running too cold can have negative effects on fuel economy. In order to maintain the correct temperature, the cooling system has a thermostat between the engine and the radiator. If the coolant’s temperature is too low, the thermostat bypasses the radiator and sends the coolant directly back into the engine.

Symptoms of a Faulty Coolant Reservoir

While a faulty reservoir can lead to an automotive disaster, most issues are easy for even the least mechanically savvy driver to discover. Diagnosing one of the following symptoms will better enable you to prevent a faulty coolant reservoir from becoming a faulty engine problem.

Your Car’s Engine Overheats

The car’s engine overheating is the worst of the possible symptoms of a faulty reservoir. It can lead to permanent damage to the engine caused by engine parts expanding and warping in the great heat. Driving an overheating car can even lead to cracks in the engine block.

Most cars have a temperature gauge on the dashboard that tracks the engine temperature. While this is not one of the gauges that need to be watched closely, keep an occasional eye on it, especially when you are driving long distances or in hot weather. Some fluctuations in the engine temperature are normal but if it moves to the upper part of the gauge, finding a place to pull off the road and give your car a break could prevent disaster.

If you continue to drive an overheating car, the next symptom will probably be steam coming from the hood. When this happens, pull over and turn off the car’s engine immediately. This will prevent further damage to your engine from expanding and warping metal within the engine. As your engine cools some cracking or ticking noises from your engine are normal.

Once your engine has sufficiently cooled, check your coolant reservoir for leaks or low levels of coolant. There are several products on the market that can fix a leaky coolant reservoir but if your car has overheated to the point it began steaming, it is best to take it to a mechanic to better determine any damage to the engine.

Sweet Smell When Engine Is Hot

While water alone can be used to fill the coolant reservoir, the vast majority of cars use a combination of water and antifreeze. Antifreeze prevents the liquid in the coolant reservoir from freezing and expanding in the metal tubes that run through and around the engine to keep it cool. This is vitally important in places where the temperature drops below freezing.

Antifreeze has a distinctively sweet odor and smelling that odor when your car is running can be a symptom that there is a leak in your car’s coolant reservoir. To familiarize yourself with the sweet smell of antifreeze, take a sniff the next time you add some to your car or, if you do not need to add any more fluid to your coolant reservoir, remove your radiator cap and take a sniff. One big caution to removing your radiator cap: when the engine is warm, the coolant reservoir is under pressure so removing the radiator cap will release steam and hot coolant. Only open your radiator cap when the engine has cooled down.

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Visual Coolant Leak

A good habit for general car maintenance is to check under your car to see if there are any stains or liquids that could have dripped from your engine or other parts of your car. While water, brake fluid, motor oil, transmission fluid, and even gasoline can be found dripping from your car, the antifreeze from the coolant reservoir is the easiest to see due to its bright green, pink, or orange color.

The best parking places to check for leaks are usually where you park your car the most, ideally one that only you use. The longer a car stays parked, the more time any fluid leak from the car has to show itself. The more public the parking space, the more likely it is that the leaks could have come from someone else’s car. If you see a leak when pulling out of a parking space at the grocery store, for example, it is better to double check that the same fluid is underneath your home.

As stated above, antifreeze can be distinguished from other leaks by its sweet smell and bright green, pink, or orange coloring. If you do find a leak of antifreeze coming from your reservoir, it is vital that you clean it up. Antifreeze has a sweet taste to go with its scent but it is poisonous to humans and animals. A puddle of antifreeze under your car could attract pets or small children.

For small leaks, the best option is to refill your reservoir and use one of the commercially available additives that can plug small leaks in the coolant system. Once you have refilled and used one of the leak plugging additive, continue to monitor your leaks and remember that it can take some time for them to stop, so further drips under your car in the days after you added more coolant to the reservoir might not be a problem. For larger leaks or leaks that cannot be fixed with additives, seek the help of a professional mechanic.

Low Coolant Levels

A leak in your car’s coolant reservoir can lead to low levels in the coolant reservoir. For most of these leaks, you will see the evidence under your car in the form of a coolant drip or a puddle, but coolant can leak inside the engine as well. If your car’s coolant light comes on or if it starts overheating, check for leaks in the coolant reservoir by looking for antifreeze puddles under your car and address them.

If no leaks can be found, it is possible that the coolant is leaking internally. In these cases, the coolant either mixes with the engine oil or is burned off in the cylinders when they combust the gas. Neither of these possibilities are good long term. The ethylene glycol in antifreeze does not work as a lubricant. Reducing the lubricating effect of the oil and antifreeze in the combustion chamber can lead to corroded or etched pistons. If you cannot determine where the leak in your coolant reservoir is coming from, consult a mechanic.


A car’s coolant reservoir is part of a vitally important system that keeps the car running smoothly at the correct temperature. Keeping an eye on the coolant level in the reservoir will ensure that this system has the coolant it needs and better alert you to any mechanical issues.