coolant reservoir

A coolant reservoir forms part of the vehicle’s cooling system and is used to store excess expanding coolant caused by heat from the engine. The cooling system relies on the coolant reservoir tank for handling overflowing coolant, reintroducing the coolant back into the system as the engine cools off. Keeping the coolant reservoir in good condition ensures efficient engine performance so that you can stay cool on the road.

How Does a Vehicle Cooling System Work?

The cooling system’s main function is to keep the engine temperature within limits. This means preventing overheating & maintaining optimum operating temperature while the engine is running. The cooling system uses liquid coolant (water + antifreeze) which is circulated through the engine block around the cylinders through water jackets to absorb excess heat that’s generated through combustion. This excess heat is then dissipated in the radiator through cooling fins. The cycle of heating & cooling is necessary for the engine to perform at its best.

Let’s get familiar with the components of a cooling system used in vehicles:


It is a large finned component that’s placed at the front of the car, in front of the engine bay. Coolant flows through a series of thin metal tubes surrounded by cooling fins. A large amount of heat is radiated to the atmosphere through the radiator fins. The coolant then returns to the engine block at a lower temperature. To facilitate efficient heat transfer, the radiator has a large fan that switches on when the coolant exceeds a certain temperature.


These are made from aluminium or other metals such as copper. The narrow hollow tubes allow coolant to be transferred from the engine block to the radiator and back (under pressure). The tubes are connected with durable rubber hoses  & clamps that keep the system airtight and prevent exposure to the atmosphere.

Water Pump:

Responsible for circulating coolant through the system. A continuous flow of coolant prevents the engine from overheating. The water pump is driven by the engine’s power via serpentine belts. As the engine runs, the water pump draws hot coolant from the engine block & pushes it towards the radiator where it is cooled. The flow of coolant is controlled by the thermostat which regulates the amount of coolant that can be pumped through the system depending on the engine temperature.

Coolant Reservoir Tank:

The coolant begins to expand as it absorbs heat from the engine. The whole cooling system is pressurised & excess coolant pours into the coolant reservoir as the engine temperature rises. Excess coolant from the reservoir is drawn back into the cooling system when the engine temperature cools down. High temperature triggers an expansion valve that pours coolant from the radiator into the coolant reservoir tank. It prevents air from entering the system & ensures there is always enough coolant to regulate engine temperature. It is mounted near the radiator and features a plastic housing with minimum and maximum markings.


It maintains the ideal engine temperature by opening and closing a valve that connects the coolant flow between the engine to the radiator. This helps the engine warm up quickly during a cold start and prevents overheating as the engine runs. Most thermostats always remain partially open to prevent catastrophic overheating.


A mixture of water and a chemical known as antifreeze which is made of ethylene glycol. It has excellent heat transfer properties that prevent it from boiling at high temperatures or freezing at low temperatures. Coolant also contains additives that inhibit corrosion and help in the lubrication of the water pump. Coolant needs to be topped up at regular intervals or flushed and replaced according to the vehicle’s maintenance. 

Coolant Reservoir: a Critical Component of the Car’s Cooling System

Also known as a coolant reservoir overflow tank, it plays an important role in preventing overheating of the engine. As the engine heats up during the combustion cycle, the coolant absorbs this heat & is circulated to the radiator to release excess thermal energy. The coolant expands with rising temperature and increases in volume as a result. As the cooling system is pressurised, the coolant would have nowhere to expand if the reservoir tank was not in place. By providing space for the coolant to expand, the coolant can continue regulating temperature and keeping the engine at optimum operating conditions.

At times when the engine is running cooler, the coolant begins to contact and create a suction force which draws excess coolant back into the system and turns the radiator coolant reservoir empty. The design of a coolant reservoir tank minimises the formation of air pockets, and bubbles and can prevent high pressure from building up in the cooling system. It is equipped with a pressure cap that can vent out excess coolant fumes at high temperatures.

Here Are Some Essential Tips to Maintain the Coolant Reservoir Tank:

Periodically inspect the coolant reservoir for signs of leak, cracks, and damage and ensure that the water in the coolant reservoir is within the recommended range (there is a min and maximum indication on it). If you notice that the coolant level is below the minimum marking, top up with water or antifreeze.

Driving without a coolant reservoir cap may cause the coolant to overflow and result in an overheating engine. Check the condition of the pressure cap and ensure proper sealing and pressure release. The cap is easily replaced.

Modern vehicles have sensors that can provide real-time data on bad coolant reservoir cap symptoms. This includes coolant pressure, temperature and status of the water pump which is automatically relayed to the ECU to provide diagnostic data for onboard scanners and to display warning lights for the driver.

If you notice any leaks from the coolant reservoir tank, address them immediately to prevent loss of coolant and potential damage to the engine due to overheating. Knowing how to clean the coolant reservoir tank is also important. Below is a step-by-step guide to keep it clean.

Turn the engine off and wait at least 15 minutes for it to cool down. Opening the cap with the coolant reservoir tank boiling could lead to hot coolant spraying, causing burns. You can syphon off excess coolant or disconnect the outlet pipe below the tank to drain the coolant.

Mix cleaning solution (detergent and water are fine), and use a narrow soft brush to gently scrub the inside of the coolant reservoir tank. Remove all traces of dirt, debris and visible deposits. Rinse with clean water to remove all cleaning solution traces (which can contaminate the coolant). Use an air gun to dry the coolant reservoir tank completely. Once all moisture is removed, replace the outlet and inlet hoses and refill coolant just up to the maximum level. Firmly replace the cap on the coolant reservoir tank.

This cleaning process can be done at every service, especially during coolant replacement or before cooling system flushing. This way the cooling system remains in good condition and contributes to the long life of your vehicle.

Bad Coolant Reservoir Tank Symptoms

Look out for these signs that could indicate that your coolant reservoir tank needs some attention

Coolant Leaks:

If you notice puddles or stains underneath your parked vehicle, it could be caused by a crack, leak or damaged cap on the coolant reservoir tank.

Overheating Engine:

When the engine temperature gauge rises to the red zone, it means the engine is overheating. Switch the engine off immediately and inspect the coolant reservoir tank for adequate coolant levels. Avoid driving the car without checking the cooling system in case this happens.

Low Coolant Levels:

Physical damage to the coolant reservoir tank will deplete coolant levels and trigger a warning on the dashboard.

Mixed Oil and Coolant:

A faulty head gasket can leak oil into the coolant which results in a frothy solution in the coolant reservoir tank.

Boiling Coolant in Reservoir:

If you notice boiling or bubbling in the coolant reservoir tank, this is a result of overheating and pressure buildup within the system.

Damage to the Coolant Reservoir Cap:

Difficulty in removing or replacing the cap could mean that it is not sealing properly or was blown off due to high pressure and blocking of the vent hose. Also, check the coolant consistency, colour and signs of floating debris caused by exposure to the atmosphere.


Understanding how a coolant reservoir tank works and how to maintain it can lead to miles of driving pleasure without worrying about your engine’s health. This is a vital component in regulating engine temperature and contributing to efficient coolant circulation. Keep your engine cool and your journeys are sure to go smoothly with a well-maintained coolant reservoir.

For more informative articles, head over to the Carorbis Blog. You’ll find the latest automotive news, how-to guides, car and bike comparisons and best reviews on car accessories.

Frequently Asked Questions

Q1. Why Coolant Reservoir Overflowing?

Ans. The coolant reservoir collects excess coolant that is caused by excessive pressure in the system due to overheating. A malfunctioning radiator cap or faulty thermostat may also cause the coolant reservoir to overflow. Once the engine cools down, the excess coolant in the reservoir is taken back into the system for circulation. Addressing the overheating issue, and radiator cap problems and checking on the condition of the thermostat may solve the coolant reservoir overflowing problem.

Q2. How Does a Coolant Reservoir Tank Work?

Ans. As the engine heats up, the coolant absorbs this heat and expands. As it increases in volume due to expansion, the excess coolant is directed into the reservoir tank through an overflow tube. The coolant reservoir tank allows for expansion and prevents pressure buildup in the cooling system. When the engine cools down, the coolant contracts and creates a vacuum which draws excess coolant from the reservoir tank into the radiator and cooling system pipes.

Q3. How To Drain Coolant Reservoir?

Ans. The coolant reservoir has a drain hose or drain valve that can be opened to release excess coolant that has been collected in it. The engine needs to be cool so that there is no built-up pressure in the cooling system. Place an empty container underneath the coolant reservoir, open the valve or disconnect the hose. This will let the coolant drain completely. Dispose of the coolant responsibly as it may contain chemicals that could harm the environment.

Q4. How to Clean Dirty Coolant Reservoir?

Ans. Drain the coolant from the reservoir and use mild soap and water together with a brush to remove all debris and dirt from it. Rinse it with water to clean it thoroughly. The plastic coolant reservoir should be cleaned of debris to ensure that the coolant remains effective. If the coolant is exposed to moisture or dirt, it may become ineffective or lead to corrosion in the cooling system pipes and components.

Q5. How Much is a Coolant Reservoir Tank?

Ans. The price of a coolant reservoir tank depends on the specific car model and type but it is usually in the range of Rs.1000 – 2000 for a replacement. There are aftermarket coolant reservoir tanks available for certain car models but it is best to use original spare parts for the best performance and efficiency.

Q6. How to Fix a Leaking Coolant Reservoir?

Ans. Coolant reservoir tanks are usually made from plastic. A leak can be fixed by applying epoxy or a suitable sealant that will plug the hole at least temporarily. This gives you enough time to drive to a service centre or mechanic and inspect the leak. As the coolant expands and heats up, it may cause more leaks in the damaged tank. You will most likely have to replace the coolant reservoir to be on the safe side.

Q7. How to Replace Coolant Reservoir Tank?

Ans. Drain the coolant and disconnect the hoses connecting the radiator to the coolant reservoir tank. Then remove the mounting bolts of the reservoir tank and take it out of the engine bay. Next, install the new reservoir tank and reconnect all the hoses with new clips. Refill the coolant and use a torch to inspect the tank for leaks. Start the engine and let it warm for a few minutes. Check for leaks again and take the vehicle for a test drive once you are satisfied.

Q8. How to Fix a Cracked Coolant Reservoir?

Ans. Cracks can be caused by extreme heat and expansion of the reservoir tank. Drain the coolant and clean the area around the tank. Apply epoxy resin or suitable sealant on the crack and allow it to cure / dry for 10 minutes. Ensure that the crack is completely covered and refill the coolant, monitoring for leaks. Replace the coolant reservoir tank as soon as possible after this repair to avoid further damage.

Q9. Is Coolant Reservoir Pressurized?

Ans. Yes, the coolant reservoir operates under high pressure when the engine is running. It needs to be pressurised to accommodate for thermal expansion of the coolant and to prevent air from entering the cooling system. If coolant is exposed to the atmosphere, it will become ineffective in cooling the engine.

Q10. What’s Causing My Coolant Reservoir to Overflow?

Ans. It is usually caused by a faulty radiator cap, malfunctioning thermostat or leaking coolant that is causing the engine to overheat and building excess pressure in the cooling system. Inspect the radiator, hoses and connections for signs of coolant leakage. Also, have a look at the water pump and ensure that it is working properly.

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