Why Is My Coolant Reservoir Overflowing? (5 Reasons Why)

The antifreeze or coolant in your car is what controls the temperature. Additionally, it is made to remain within a closed system and is highly poisonous.

So, if you see an overflow, it can be the result of a broken radiator cap, water pump, thermostat, or radiator. Join me in discovering more!

Why Is My Coolant Reservoir Overflowing?

Your coolant reservoir is overflowing due to several reasons, such as a damaged radiator cap. If you have a loose or cracked cap, it will prevent coolant from staying in place, resulting in overflowing. In addition, a faulty water pump or seal could also be the reason behind your coolant reservoir overflowing, among other reasons. 

Learn more about why your coolant reservoir is overflowing. Here are five reasons why. Keep reading for more information!

1. Damaged Radiator Cap

This should be checked first since it is cheap and simple to replace. Your issue can be a loose or cracked cap that is preventing coolant from staying where it should be causing it to overflow.

Additionally used as pressure relief valves are radiator caps. They will stop too much pressure. Unnecessary high pressure may harm the water pump seal, hoses, heater core, and radiator. 

Likewise, the pressure cap stops radiator hoses and storage tanks from breaking. A vacuum is created in the cooling system as the engine cools down. 

Instead of air, the coolant is drawn back into the system by this vacuum preventing the coolant from overflowing.

Therefore, when performing routine maintenance, always inspect the cap. Never open the cap while the engine is still warm. 

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Put a towel over the cap and remove it with the engine off. Until it hits the safety limit, turn it 1/4 turn counterclockwise. 

Before taking off the cap, press down on it and rotate it counterclockwise to let all the pressure out.

2. Faulty Water Pump And Seal

You can lose coolant as a result of overflowing due to a poor water pump seal. Your water pump’s stability depends on this seal, which holds it in place. 

The seal is often made of rubber, which can harden and shatter with time. As a result, the coolant begins to flow from it.

Of course, checking the water pump for any potential leaks is a smart idea if your coolant reservoir is overflowing. 

It is better to replace the seal if there are any. It’s a smart idea to completely replace the water pump while you’re repairing the seal.

You’ll save money in the long run since a water pump servicing is advised every 60,000 miles. 

Get a different water pump to replace your old one if you’re close to that mileage, and you won’t pay a lot for a new seal.

3. Damaged Radiator

Damaged Radiator

If the radiator cap, thermostat, and water pump all appear to be in working order, the reservoir is probably overflowing due to a radiator issue. 

The radiator in earlier cars was constructed of aluminum and copper. The radiator in more recent cars is largely made of plastic. 

Although plastic radiators are lightweight, they typically wear out more quickly. If the leakage is discovered, it is advised to replace the radiator rather than fix it.

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4. Head Gasket Leakage

One of the problems you generally don’t want to face with your car is a burst head gasket. To be honest, head gaskets could be annoying. 

Therefore, avoiding engines that are susceptible to head gasket breakage is the best course of action.

So, a coolant reservoir problem will arise for you. As soon as there are a few holes within the gasket and the automobile starts leaking coolant, this reservoir will begin to overflow and drain. 

The car will frequently overheat if the coolant reservoir is not topped off, and the car continues to lose coolant.

Therefore, the heat will cause the engine head to begin to distort; if it does, the engine head is likely to be destroyed. The best course of action is to purchase a replacement head. 

And if the head is not twisted too far, a simple resurfacing will solve the problem. If an engine head requires excessive machining, repairing it might run you $1,000.

5. Bad Thermostat

The majority of the other issues will make the check engine light come on, but there may be one that the computer in your car is not aware of. 

Inadequate fluid regulation by a malfunctioning thermostat might result in an overflow. Keep in mind your car has the appropriate level of coolant. An overflow could happen if you overfill.


Overall, we’ve discussed the issue of the coolant reservoir overflowing in particular. To better understand what you need to troubleshoot, you must be aware of these things.

Now that you know what to look out for and what to do when the coolant reservoir overflow problem arises, you can deal with it effectively.

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