Symptoms of a Failing Coolant Expansion Tank

The coolant expansion tank in your car is a part of your cooling system. The cooling system is responsible for keeping your engine at the correct operating temperature. It does this using a pump to circulate coolant through the engine. Coolant is a fluid that is a mixture of antifreeze (ethylene glycol, a form of alcohol) and water. As the coolant circulates, it absorbs the intense heat generated by the combustion process in each cylinder.

The coolant’s path then takes it through your radiator, where it releases the heat it absorbed as it traveled through the engine. This continuous process keeps your engine at its optimal operating temperature, for smooth running, good fuel economy, and proper functioning of the emission controls.

Your cooling system is pressurized, which allows the coolant to protect your engine from higher temperatures than if it were not under pressure. Your coolant expands under this heat and pressure, and it needs a place to go. This is what your coolant expansion tank is for.

The coolant expansion tank, located under the hood next to the engine, acts as a temporary overflow container for this “extra” coolant. After you shut off your car and the engine cools down, the coolant contracts and returns to its normal location, inside the engine and the radiator. This system also prevents air from entering your cooling system, which can result in harmful rusting.

The translucent plastic coolant expansion tank is also usually the location where additional coolant is added when needed. Most coolant expansion tanks today are pressurized, like the rest of the cooling system, the cap should never be removed while the system is under pressure. There are marks on the coolant expansion tank indicating “low” and “full,” so that you will know the correct coolant level and when to top it up. For safety’s sake, add coolant only when the engine is cold.

What are the signs of a bad coolant expansion tank?

If you do drive a car that has a coolant expansion tank, there are several ways that you can tell whether you have a bad coolant expansion tank. Let’s see what they are:

• Leaking coolant: If you should notice drips or puddles of coolant under your car, near to the location of your coolant expansion tank, this could be an indication of a bad coolant expansion tank. Call your mechanic for guidance and have the leak fixed promptly.

• Empty or cracked coolant expansion tank: If upon opening the hood, you see evidence of cracks in your coolant expansion tank, or the tank is completely empty, look for a corresponding leak on the ground underneath. Try topping up the tank with coolant and see if the coolant leaks out. Call your mechanic with the results and ask if you should drive your car over, or have it towed.

• Leaking or damaged connecting hoses: There are often narrow rubber hoses that connect the coolant expansion tank to the rest of the cooling system. If these are cracked or broken, they will allow coolant to leak out. Take a close look and see if you can detect any leaks or faulty fittings. Top up the coolant if it is low and check again. Contact your mechanic about what to do next.

• Low coolant level: Even if you don’t see any leaks or cracks in your coolant expansion tank, there can be another source of coolant loss. If you are frequently topping up your car’s coolant, get it to your mechanic’s shop and have it fixed, before it gets worse and your engine overheats.

• Low coolant warning light: If you are consistently losing coolant because of a bad coolant expansion tank, your low coolant level dashboard warning light may illuminate to alert you. If this happens, pull over right away and shut off your car. Allow the car to cool down before opening the hood, check the condition of your coolant expansion tank and add coolant if it looks like it will hold any. Either way, give your mechanic a call and get advice on next steps.

• You smell coolant: Your coolant has a distinctive sweet smell that is different from any other automotive fluid. If you smell it, either inside or outside of your car, it could be a sign of a bad coolant expansion tank. Check the condition of your expansion tank and consult your mechanic on what to do next.

• Overheating: This is the most extreme symptom of a bad coolant expansion tank. If your coolant expansion tank is no longer capable of holding coolant, and you are not aware of it, a significant amount coolant will leak out of the system, onto the ground. This means that there will not be enough coolant left in your cooling system to absorb and remove the intense heat of the combustion process. Your engine will run hotter and hotter, until something fails. If things get to this point, you can face some very expensive repairs, or even the end of your car’s life.

Should you drive your car with a bad coolant expansion tank?

Definitely not. Driving a vehicle with a bad coolant expansion tank can result in overheating, as well as failure of the cylinder head gasket, engine block, or cylinder head. An improperly functioning cooling system, if neglected, can easily result in catastrophic engine damage.

How do you fix a bad coolant expansion tank?

As with any cooling system repairs, your car should always be allowed to cool down to ambient temperature first, to avoid the potential for burns, scalding, and other problems of working on a pressurized system.

It is much easier for the mechanic to replace your bad coolant expansion tank with a new one. A brand-new coolant expansion tank is not expensive. Your mechanic will empty the old tank, remove it from the vehicle, and install the new tank. After refilling the coolant expansion tank with coolant, the mechanic will start the engine, and then check that there is the right amount of coolant in your cooling system.

Can you replace a bad coolant expansion tank yourself?

If you have a thorough knowledge of and experience in working on automotive cooling systems, you should be able to manage the replacement of a bad coolant expansion tank.

It is essential, though, that you are diagnostically proficient enough to know whether the bad coolant expansion tank is the only source of your problem. If it is, replacing your bad coolant expansion tank should resolve the situation. If it is not, there could be other cooling system issues left unidentified, and these could lead to engine overheating if not dealt with.