Do You Add Coolant to the Radiator or Reservoir?

One of the most important components of the engine is the cooling system. It plays the important role of ensuring efficient engine operation and preventing overheating. Adding the coolant is therefore an important maintenance service.  

When your coolant levels run low, it is advisable to add more as soon as possible. The new and correct coolant for your engine should be added to the reservoir, and not the radiator. You are required to loosen the reservoir cap, and fill the coolant to the cold fill line.

This article explains the correct procedure to be followed when adding coolant to your car. Read on. 

Do You Add Coolant to the Radiator or Reservoir?

add coolant to radiator or reservoir

Typically, when the engine is running, it not only produces power but also heats. Sometimes, the heat may be too much and this causes overheating. That is where the coolant comes in—it is instrumental in cooling the engine as it circulates through so that the engine maintains optimum operating temperature.

On top of that, the coolant contributes to the efficiency of the engine in that it reduces emissions as well as provides heater air on cold days. 

With that in mind, it is important to ensure that your vehicle always has the right amount of coolant. You can do so by checking it every time you go for a fuel refill. What many do not know is that there is a right procedure to add coolant to a car.  

How to Add Engine Coolant to Your Car.

Before you do anything, it is important to first refer to the owner’s manual on the right coolant for your engine. Different engines are compatible with different types of coolants. 

Ensure that your engine is turned off before opening the hood. Then, locate the engine reservoir—it is usually translucent and white in color with hoses that connect to the radiator. 

Once you have the right type of coolant, ensure that you do not have any loose clothing, hair, and avoid the cooling fans at all costs. 

Adding coolant should never be done when the engine is still hot as it is likely to cause serious damage. Once your engine has cooled down, place a thick cloth, preferably a towel, on top of the reservoir cap before removing it. 

Afterward, loosen the cap a little and step back to allow some of the pressure to be released. After that, you can completely remove the cap. 

The coolant should be added into the reservoir until the cold fill line. Then, replace the reservoir cap and tighten it. 

Reservoirs have a fill range calibrated on the side. If your engine is sufficiently cold, the level indicated should be on the cold fill line. 

Once you are done, close the hood and turn on the engine to allow the coolant to sufficiently circulate throughout the engine. 

What Are the Signs of Low Coolant Levels?

If your car engine is low on coolant, there are some tell-tale signs that should warn you. 

Smoke From the Engine

You may notice some white smoke from under the hood or the exhaust of your vehicle. This means that the coolant runs have run dangerously low and thus cannot perform the task of cooling the engine. This often happens when the engine overheats. You should refill the coolant as soon as possible. 

Overheating of the Engine

This is one of the most common signs that indicate an issue with the engine cooling system. It could result from a faulty radiator, a bad thermostat, or simply a low coolant level.

This problem should be diagnosed and fixed sooner rather than later as it can significantly reduce the functionality of your vehicle. It could also cause a sudden breakdown of the car. 

Leakage of Coolant 

Another way to diagnose low coolant levels is if you notice leaking coolant from your car. It can leak from either the radiator or connecting hoses. 

What Is the Function of the Coolant?

The cooling systems circulate the coolant through the engine heads and blocks. This coolant absorbs heat in the engine surrounding as it passes through. With time, the coolant gets heated and then flows to the radiator.

It is transmitted through a rubber hose where the hot coolant circumnavigates through the thin radiator tubes. In the process, is cooled by the cooler air coming from the car’s grill to the engine compartment. When cooled, the coolant flows back to the engine. The process is repeated with the aim of reducing the temperature.  


The radiator reservoir is invaluable in the engine cooling system. It is important that the coolant level in the reservoir is at the maximum line at all times. This reservoir works to store excess coolant until it is needed to reduce the temperature. If the levels are low, the engine is likely to overheat. 

The coolant reservoir is likely to leak in case of any damage. This can cause it to constantly drip coolant and thus completely empties the reservoir. Moreover, if the reservoir tank is damaged, it can lead to the evaporation of the coolant. Hence, any time you add coolant, it never fills up. The drop rate of your coolant will be directly proportional to the extent of the damage. 

Sometimes the hot coolant from the overflowing reservoir does not return to the radiator. This can be due to leakage or a clog in the radiator hoses that allow the coolant to return. Also, it could be because of a damaged radiator cap as it allows the escape of the coolant. 


It is important that the engine is supplied with adequate coolant at all times. This coolant should be added to the reservoir and not directly to the radiator or the engine. Drivers and car owners are advised to monitor the coolant levels as frequently as possible.

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