How Much Coolant Should Be In The Reservoir? Here’s The Truth

Your car’s cooling system is vital to its function. Without the engine cooling ability, you can’t drive your car for too long without creating damage. Problems related to Coolant might completely make your car useless within some minutes, therefore, you need to know how to diagnose overflow tank issues and other coolant-related issues as well.

Read on to find out about how full your coolant overflow tank should be, how do you fill the coolant reservoir all the way, and more you need to know about your coolant reservoir.

driver checking coolant

What is coolant?

Car coolant, also called antifreeze, protects the engine from overheating. Also, the Coolant lubricates every moving part it gets in contact with, protecting against damage to the water pump, head gasket, cylinder, and piston timing.

Every automotive coolant is glycol-based. Normally, coolants include an ethylene glycol mixture with some water and additive packages. Another glycol-based coolant includes water and propylene glycol. The main difference between these two types is the propylene glycol being less toxic.

The heat-carrying ability of pure water is more than the pure ethylene glycol, therefore water is the most suitable coolant if you are only considering the ability of coolant of carrying off the heat.

However, water gives other difficulties. It produces rust on the iron parts of the engine. Then, the rust is taken off to the other cooling spaces. Heat transfer interferes with the resulting corrosion even before build-up plugs the radiator and supplies sediment to the cooling system.

Auto repairman pouring coolant into the radiator.

Coolant helps in reducing engine rust and corrosion. Coolant further gives resistance to freezing. It will not freeze and extend in hyper colds as the water would. That guards the engine against cracking and undergoing increased pressure.

Moreover, if the coolant boils, the vapors produced don’t transfer the heat well, which means that the engine metal actually can melt if the coolant is not kept in contact with some places that should stay cool. Some of the modern vehicles are built with tight engine parts which don’t feature sufficient airflow, which means they might overheat in a few minutes if a functional cooling system is not there.

Engine coolant is used in combination with the liquid cooling system. That liquid cooling system is created from several components.

  • The coolant pump, or water pump, circulates the coolant everywhere in the system.
  • Radiator carries heat off the coolant.
  • The radiator hoses combine the cooling system’s parts.
  • The fan pulls the air through the radiator if the vehicle isn’t moving fast enough for moving the air.
  • The thermostat checks the coolant’s temperature.

Additionally, the Coolant serves the purpose of guarding the metals and non-metallic elastomers (like plastic and rubber parts) in the cooling circuit and the engine. If your system doesn’t have a proper coolant system, component and corrosion damage could lead to long-term effects. They are seldom latent, which means it takes around 12 months for corrosion damage, deposits, and plugging to give a rise to a problem.

Oftentimes this is misidentified by the drivers as a failure of the radiator instead of simply thinking that it’s the fault of a wrong coolant. In the case of a radiator ending up corroded badly or full of plugging internal deposits, a coolant system that’s not working properly is possibly a cause.

And because the problems related to coolant occur inside the motor, you may not realize the causing damage unless you have a look at cooling passages and internal heat-transfer surfaces of your car’s engine.

What amount of coolant should be in an overflow tank?

A container storing excess or overflowing coolant is called a coolant reservoir and sometimes an overflow tank. It is commonly a transparent container of plastic near the radiator. The overflow itself is where the coolant is directly poured and not in the radiator as most people believe.

As mentioned earlier, engine coolant carries away excess heat from inside of the engine. It is a mixture of antifreeze and water. When the coolant level gets very low, it creates extra heat which can damage the engine.

You might question how much amount of coolant should you put in the car. Read on. You will find that out in easy words. Coolant level checking is quite simple.

After opening your bonnet you must locate the coolant overflow tank. Your radiator is connected with a water hose close to your radiator cap connecting to your overflow tank. It can be found out relatively easily. The coolant inside could be gold color, yellow pink, green, orange, or red. Don’t take the wiper reservoir for your coolant reservoir.

On the overflow tank body, you’ll find 2 marks, Low and Full. These lines show if the coolant is high or low. When coolant is beneath the mark of “low”, you must add extra coolant to your reservoir.

Overflow tank filled with coolant to the required level.

Before adding more coolant to your overflow tank, it’s imperative to ensure that the engine is not hot. If your engine is hot, there might be pressure in the reservoir, and when opening it could cause severe injury by splitting out. Make sure that there is not any pressure trapped in it.

For opening your radiator or reservoir, put a rag on the cap when opening it to prevent a split of water on you. The amount of coolant that has to be in the overflow tank should be enough – this means the level of coolant is maximum.

If you are asking, how much coolant should be in the radiator? Unlike overflow tank having a low and full level indicating when coolant is either overfilled or low, the radiator doesn’t have maximum or minimum levels. You have to simply fill the radiator to the brim.

How do you fill the coolant reservoir all the way?

In older days (usually before the 2000s), changing the coolant of your vehicle was a simple task. After 2000, the cars started getting more complex and a lot of DIYers got intimated by precise filling and “air bleeding” methods needed for eliminating the engine air pockets. But do not worry, it is easier than it seems. We’ve sorted out everything for you in 6 easy steps.

But before getting into the steps, let’s first have a look at everything required on-hand before getting started.

It’s recommended to use a 50/50 mix of coolant and water. Check your manual of the owner for making sure that you’ve got the appropriate coolant. A funnel can help in avoiding spillage, however, it is not essential.

6 Easy Steps to Top Off Your Coolant:

Step 1: Letting your car rest.

Park the car on the ground level and allow it to sit there for some hours without driving to make sure it is not extremely hot.

Step 2: Opening the hood to locate your coolant reservoir.

If you are one of those people who have not done it before, you will need to find where to put the antifreeze in the car. Find the reservoir while you hold the coolant; it is normally a plastic tub having inside of it a colored liquid. The cap has to be marked with such a warning as “Hot” or “Caution”.

Step 3: Checking the level of your coolant.

A coolant reservoir has to be labeled, and they’re normally translucent plastic. The reservoir’s outside has to be marked with maximum and minimum lines for your coolant, the need for refilling the coolant can be checked here. Referring to your owner’s manual is recommended if you are not sure of locating the coolant reservoir.

Step 4: Mix the water and coolant.

If your coolant isn’t pre-mixed already, use some empty container for mixing equal parts distilled water and coolant for creating the 50/50 water and coolant mix that is recommended usually. Normally higher quality coolants are the longer life ones. Always use high-quality coolants and get the most reliable results for the engine.

Step 5: Topping off the radiator.

If you cannot find a coolant in your radiator and it holds a cap that can be accessed, go forward and top off till you see a coolant at the neck’s bottom. If some air is trapped inside, it can rise up to the surface, which can cause the level to drop. In such a case, simply fill up it again and put on the cap back.

Step 6: Topping off the coolant reservoir.

Same as the radiator, the coolant reservoir will be shown with maximum and minimum lines. Pour the coolant in till it touches the maximum line, and then put on the cap back.

What if the coolant is too full?

The reservoir designed for accommodating the natural contraction and expansion of coolant is known by the name coolant tank and sometimes anti-freeze tank. Coolant expands while it heats and contracts after cooling. The additional space prevents the engine and hoses from any damage.

Unlike the oil dipsticks with one mark, the “maximum” level, the coolant reservoir has two marks. These marks gauge the coolant level in your vehicle when the temperatures of the engine are different. The lower of these two marks are used if the engine is cool, whereas if the engine is hot another mark is used.

The cooling system of your vehicle is made for accommodating a little quantity of extra coolant. Mostly, the overflow hose expels the excess coolant. You will likely notice a coolant’s puddle below your car after it happened.

In worst-case scenarios, overfilling the antifreeze tank could lead to electrical damage when overflow comes in contact with the wiring of the engine. The overfilling might come with some outcomes.

Possible Outcomes of Overfilling the Coolant Reservoir:

Coolant spill on the tarmac.

Damage to the Environment and the Stray Animals

Mostly, when a coolant reservoir is overfilled, its excess is drained out by the overflow hose. Your car might not suffer from any issue as such, however, the expelled coolant on the floor can harm the animals alike and the environment.

Coolant tank with warning information.

Pressure Build-Up

When a coolant reservoir is overfilled, there will not be many areas available for the expansion of the heated coolant. Ultimately, the pressure in the reservoir would gradually increase. In such a case, the tank cap will open, and hot coolant would burst out like the puddle of some lava.

Also, there may arise a case where the tank cap would be non-functional owing to its tightening or corrosion. In such a case, the build-up pressure of the overfilled coolant reservoir would burst out from the freeze plug.

Car engine wiring.

Electrical Damage

That’s one of the most critical scenarios that can happen because of extra coolant in your reservoir tank. Chances are very rare, however, the overflowing coolant might get into contact with the engine’s electrical wiring. When this happens, electrical failure might occur.

Moreover, the electrical damage will lead to the car not starting, headlights dimming when driving, leakage of battery, or malfunctioning of brake lights. If any such kind of symptoms happens, your visit to an expert mechanic would help.

Knowing the damages that can occur because of the overfilled coolant reservoir, all that’s required is being cautious. Be careful always while filling the coolant reservoir or antifreeze and remember to fill it simply to the levels marked. Such steps can help in saving the car, the money, the animals, and the environment.


Coolant is an important component of your car which serves your car as a cooling system. This simply indicates that you can’t go for more than 5 minutes without it since it can seriously damage your car and the engine. It’s a rather simple substance with a big responsibility.

From this article, you have learned how much coolant should be in the overflow tank and other related problems. With the efforts put in this article from our end, we hope this will help you in taking care of your coolant reservoir and your car more properly. If you still think something is missing in this article, feel free to leave a comment. Your comments matter to use.

Good luck to you and your car!

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About Matthew Webb

Hi, I am Matthew! I am a dedicated car nerd! During the day, I am a journalist, at night I enjoy working on my 2 project cars. I have been a car nerd all my life, and am excited to share my knowledge with you!

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