Why Is The Coolant Reservoir Overflowing? (+ Fix Explained)

Coolant or sometimes called antifreeze is a vital fluid for your vehicle. It regulates and maintains the temperature of your vehicle. This will keep your engine from overheating. 

Unfortunately, coolant can also be harmful, so that is why it stays only in a closed system. However, there are situations where the coolant reservoir is overflowing. The overflow can be caused by a radiator cap, water pump, thermostat, or even a radiator malfunction. 

So if you have a coolant reservoir that is overflowing, this article is meant for you. Have fun reading.

Why Is The Coolant Reservoir Overflowing

What’s Causing My Coolant Reservoir To Overflow?

If you have a coolant reservoir that is overflowing, here are the possible causes. 

1. Coolant Level

You have to ensure that the coolant in your coolant reservoir is the proper level of your coolant. If you overfill it, it can overflow. 

2. Radiator Cap

You need to check if your coolant reservoir is overflowing. You may have a loose cap, or you might have a damaged cap. This is where the coolant should be. 

The radiator cap is made to give access to the fluids in a closed system. The coolant will cool down the engine. You can check your owner’s manual for a proper cap replacement. 

3. Water Pump 

You might be having a faulty water pump. It can slow down and put a stop to the flow of the liquid into your cooling system. 

As a result, you will have an overheated engine. You can also look for coolant in the ground and around the water pump.  

4. Thermostat

Some of these problems cause a check engine light to turn on. But you might be having issues, and you will not notice because the system of your vehicle has not noticed it either. You can have a bad thermostat, and it will not correctly maintain the fluid in your vehicle, and it can then lead to overflow.

5. Blown Head Gasket

If you see a gas bubble, or you might notice that there are boiling in the liquid if you take off their radiator cap, you might have a cracked head gasket or a blown head gasket.

6. Radiator

The reservoir can be overflowing if you have a radiator problem. The radiator is made up of copper and aluminum. 

The radiator also has plastics for modern vehicles. You need to replace the radiator so that you can fix the leak. 

How To Fix The Overflowing Coolant Reservoir?

If you want to fix your overflowing coolant reservoir, follow these steps. Look for a broken or defective radiator. If your radiator cap has gone bad, it will allow many coolants to pass by and overflow. 

It is the cheapest part to replace, so have it replaced. Next, you have to check for the thermostat. If your thermostat has gone bad, the radiator will not regulate properly. 

The amount of liquid will then pass by too little or too much. Have it replaced as needed. Finally, you have to check for the water pump. 

Replace it if needed. Replace also the radiator if it is faulty. 

Coolant Reservoir Overflowing And Overheating: What To Do?

You need to find out first the cause of the overflow in the coolant reservoir. Then, if you have found out the cause, have the part fixed or replaced. 

But you can fix something that you do not know. This is why it is essential to know first the cause of the overflow. Only then can you fix the problem. 

Coolant Coming Out Of Reservoir Cap: What To Do?

If your cap is not releasing excess pressure in the right way, the pressure can compel the coolant to leak out. It can be from the cap, the engine gaskets, the hose, the water pump, and even your vehicle’s radiator. 

If you know the problem, consult a mechanic. They will fix the problem, or they will replace some parts. 

Coolant Reservoir Overflow After Driving: What To Do?

You need to check for a broken or defective radiator cap. If the radiator cap has gone bad, have it repaired or replace the parts. 

You also need to check for the thermostat. Check for the water pump and check the radiator. Have the parts replaced if needed.

Coolant Reservoir Overflowing In Honda Civic: What To Do?

Replace parts if needed. The water pump might not be operating correctly. The coolant reservoir can also overflow. 

There might be too much coolant that is being pumped. Have it replaced after consulting an expert.


In summary, coolant is essential for your vehicle. It is a problem if your coolant reservoir is overflowing. 

With that, you need to have your vehicle fixed. Overflowing is not also healthy for your engine.



Image credits – Canva

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My name is Hank, and I've been in the automotive industry for 27 years. I've been working in my own auto repair shop for the last 13 years, and now I want to help you here, on my blog. Let me know if you have any questions. Read more