Coolant Expansion Tank


Coolant Expansion Tank

The cooling system of a vehicle is a complex network of hoses, electronic devices and some other components. One of them is the coolant expansion tank. Along with other cooling system parts it helps the engine to run within its optimal temperature range. Everything starts with a pump that causes the coolant to circulate through the engine. Being a mixture of antifreeze and water, the coolant perfectly absorbs heat generated during engine operation.


The route of the coolant goes through the radiator where the heat absorbed from the engine is released before the coolant returns to its start point. This process continues again and again while the engine is running, preventing it from overheating and maintaining its optimal temperature to ensure good fuel consumption, low emissions and high performance.

The pressure inside the system is usually higher when the engine is running. This allows for protecting the engine from significantly higher temperatures. Being pressurized and warmed up, the coolant expands, so it needs an extra place to go. This is why your engine cooling system contains a coolant expansion tank. As a part of the system it provides that extra space for the coolant when needed. In about half an hour after you stop the engine it will cool down and the coolant will return to its normal location. The use of the coolant expansion tank prevents air from entering the system as well as allows the engine cooling system to operate within its normal pressure range.

The majority of coolant expansion tanks are made of translucent plastic and located where the additional coolant can be added if needed. As a rule, an expansion tank is also pressurized, so the expansion tank cap should never be removed while the engine is extremely hot and the system is under high pressure. It features “min” and “max” marks on the front side, so you can check the coolant level and top it up if necessary. 

Symptoms of a bad coolant expansion tank

If your vehicle is equipped with a coolant expansion tank, there are a couple of signs which can tell you the tank should be replaced.

Leaks: In the event you notice any drops or spots of the coolant under the car in the area of the expansion tank, this may be a sign of a leaking tank. If so, get in touch with your mechanic to fix the problem.

Cracked and/or empty tank: If the tank is empty or you notice any cracks on its walls, inspect the space under the car for leaks. You may top up the tank with new coolant to see if it leaks. If any leaks appear, call your mechanic and ask for your next steps. 

Broken or leaking connecting hoses: The hoses that connect the coolant expansion tank to the rest of the system are usually narrow and can get cracked or broken. Inspect all the hoses to determine any signs of leaks, damage or faulty fittings. If no problem is found add some coolant to the tank and check the system again. 

Coolant is under the “min” level: If the coolant level is low, but you don’t see any leaks, damaged hoses or other engine cooling system parts, there can be another source of the problem. In case you have to add coolant on a regular basis, contact your mechanic to determine the problem and fix it. 

Low coolant warning light is on: If your dashboard is equipped with a low coolant warning light it may come on because of a low level of the coolant in the system. If you see the light, shut down the engine and wait until the car cools down. Then open the hood and check the level of the coolant and the condition of engine cooling system parts such as the coolant expansion tank, hoses, etc. Even if everything looks good, get in touch with your mechanic for instructions.

You notice a sweet smell: As a rule, coolant has a sweet smell unlike any other fluids in a vehicle. That’s why if you smell it, this could be a sign of a leaking coolant expansion tank or another problem related to the cooling system. If you don’t see any leaks and the coolant level is good, call your mechanic for a piece of advice.

Engine overheating: This is the first sign you should pay attention to when driving a vehicle. If your coolant expansion tank is damaged and leaks or there are any other leaks in the system, the engine will quickly get overheated. Due to a low level of the coolant the cooling system will not be able to absorb and release the intense heat, causing the engine to run hotter and hotter. In the end this may lead to serious troubles which will cost you a pretty penny.

May I drive a car with a bad coolant expansion tank?

The answer is definitely not. Any problem with a cooling system is a high-way to engine overheating and, as a result, to its damage including failure of the cylinder and engine blocks, and their gaskets.

Is it possible to fix a bad coolant expansion tank?

Before beginning any engine cooling system repair you should allow your engine and car to cool down to avoid any potential injuries.

A coolant expansion tank is usually not expensive, so it’s better to replace your failed one with a brand-new tank. The installation process is quite simple. Your mechanic will remove the old tank, install a new one, refill the system with coolant and start the engine to check whether additional coolant is needed. 

Can I replace it myself?

Since the installation process is not difficult you can try to do the trick yourself if you have a thorough knowledge and skills.

Keep in mind that you should be diagnostically proficient enough to find out whether a bad coolant expansion tank was the only source of the problem. If other issues exist and they are left unidentified, they may cause another cooling system problem and engine overheating.  

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