Causes of Coolant Coming Out of the Overflow Tube

The primary causes of coolant coming out of the overflow tube are overfilling antifreeze, defective radiator or a bad radiator cap / thermostat. Overfilling the coolant tank will cause an overspill as the vehicle tries to find a balance. Also, using a weak radiator cap or the wrong one may result in an overflow.

The cooling system is a vital part of the engine and is responsible for its temperature regulation. However, coolant coming out of the overflow tube may indicate more serious issues with the car, including the engine overheating.

Suppose you are experiencing this issue – don’t worry! This article outlines all you need to know about fixing a coolant overflow.

coolant coming out of overflow tube

Why Is Coolant Coming Out of the Overflow Tube?

Overflow of coolant from the overflow tube or hose is not normal and should be treated with utmost seriousness. Coolant overflow can be caused by many factors, including overheating problems, damaged radiator caps, overfilled antifreeze, faulty radiator and water pump issues.

Below are some common reasons your coolant may come out of the overflow tube.

Damaged radiator cap:

A significant cause of coolant overflow is a radiator cap that cannot keep the coolant sealed. It could be because you have cheap radiator caps or the lid does not fit properly. Some other reasons may be the result of a cracked cap.

The radiator cap is meant to be tightly closed to keep the coolant’s pressure intact.

Coolant works through a closed system allowing pressure to build up, therefore allowing movement of the coolants. However, when the pressure escapes, there will be no efficient movement of the liquid, causing it to overflow through leaking parts. 

Engine running hot:

Additionally, the coolant will come out from the overflow tube if your engine is running too hot. When the heat of the machine is beyond the average working temperature, pressure builds up in various parts of the engine, including the coolant reservoir and radiator.

When pressure builds up in the coolant or radiator, the coolant will rise to the caps and overflow the hose, causing it to spill out. It is risky as coolant is flammable.

There are many reasons your engine could run too hot even with the right amount of coolant in the reservoir. Some may include damaged radiator fans, leaks in the cooling system, and more.

Ensure that your vehicle’s cooling system works well when you encounter coolant coming from the overflow tube.

Damaged water pump:

A water pump is essential in any vehicle’s cooling system and how the coolant functions. The water pump controls how the coolant liquid circulates throughout the engine system to absorb extra heat.

Once the water pump is damaged or does not work correctly, the coolant will not be able to move as it should. When this happens, engine overheating is imminent, and you begin to see coolant where it is not meant to be.

Overfilled coolant level:

It is essential to understand that each vehicle has a specific level of coolant capacity. When you exceed this level, your coolant will spill to other parts of the engine, and that is when you see it coming out of the overflow tube.

To avoid this problem, consult your vehicle’s manual to get the coolant capacity for your engine. 

Leaking gasket head:

Another common cause is a leaking gasket seal or head. When there is a problem with the gasket head, pressure can also build up in the radiator, causing the coolant to overflow and white smoke under hood.

To check if the gasket head is the cause of your coolant getting out through the overflow tube, you will see a bubble in the coolant when the radiator cap is open. Do not perform this test yourself as it causes hot coolant to shoot out of the radiator when opened – seek professional help.

What Causes Too Much Pressure in the Cooling System?

If you experience too much pressure built up in the cooling system, you have an overheating engine problem. Heat is the primary cause of pressure build-up in most engine system parts that store liquid.

When there is excessive heat in the engine, the coolant’s reservoir gets very hot, which causes the coolant to expand. When this happens, pressure is created in the enclosed area.

Too much pressure in the cooling system can cause many problems, including leaks in coolant pipes, cracks in caps or seals, coolant spills, and much more.

When you experience too much pressure in the cooling system, you must get the cooling system checked immediately. A proper problem diagnosis is needed, as well as a proper fix done. 

Can a Bad Thermostat Cause Coolant to Overflow?

A bad thermostat can cause your coolant to overflow. It can happen when the thermostat does not accurately regulate fluid flow.

The thermostat is an essential piece of equipment for the cooling system. It is a valve that controls coolant that flows through the engine system during the cooling procedure.

When the thermostat is damaged, the thermostat valve cannot open up to allow coolant to flow into the engine system. Instead, it causes pressure to build up in the coolant reservoir. The pressure will then cause the coolant to spill from any coverings.

coolant reservoir cap leak

What Happens if the Radiator Cap Pressure Is Too High?

High radiator cap pressure can cause severe damage to the parts of the cooling system. Parts like the radiator water pump, tubes, and more aspects of the cooling system can be damaged.

Many people do not understand that the radiator cap plays a significant role in ensuring the cooling system works perfectly.

A vacuum is created when the coolant circulates throughout the engine system, which backs all the coolant into the radiator. The coolant will fly through the air if the cap is not working perfectly. The lid should be checked regularly to ensure the pressure is not too high to avoid damaging the cap.  


When you notice that coolant is spilling out from the overflow tube, it must be taken seriously, and the underlying cause must be solved immediately.

When this issue comes up, it shows that your vehicle is overheating and its cooling system is failing. It is advised to visit your local professional to get the problem solved.


Mr. Shafiqule Islam is a graduated Mechanical Engineer and has more than 15 years experience of repairing and maintenance of different brand vehicles like Toyota, Mitsubishi, Ford, Mercedes, BMW etc. He is also giving training to Mechanics. He has started writing to share his practical knowledge to Vehicle Owners, Drivers and Mechanics to keep their cars at best fit.

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