Causes of Radiator Cap Won’t Come Off – How to Open

A defective radiator cap can cause severe engine damage, irrespective of how insignificant most car owners think the component is. If your car’s radiator cap won’t come off after the last time you topped off the coolant, you risk an engine problem.

The cooling system plays a vital role in the smooth running of your vehicle. Meanwhile, the radiator cap and other related components work in synergy to achieve the entire engine cooling process. In other words, no part is insignificant.

So, if you have a stuck radiator cap and you’re contemplating how to open a radiator cap, kindly dedicate a few minutes to digest the information in this article in order to resolve the problem as soon as possible.

coolant reservoir cap won't come off

What Does The Radiator Cap Do?

The radiator cap is responsible for regulating a vehicle’s cooling system pressure while keeping the system sealed or protected from external contaminants. The component also allows coolant to travel to the expansion recovery tank once the fluid gets hot and expands, and the pressure increases.

The radiator cap maintains an engine’s optimal temperature. While sealing the top of the radiator, the component creates a closed cooling system that enables the circulation of coolant throughout a car’s engine.

The component operates at a specific pressure which is determined by the manufacturer’s specifications. This is usually indicated on the radiator cap itself. 

The pressure rating is crucial in maintaining the fluid’s boiling point at a high temperature, which is essential for efficient heat transfer as well as the prevention of engine overheating.

Furthermore, the radiator cap also serves as a safety valve, which allows excess pressure to escape quickly in a case whereby the cooling system gets over-pressurized. Ultimately, this helps to protect the engine from any form of damage that can lead to engine overheating.

Causes of Radiator Cap Won’t Come Off

While it can be challenging to experience a stuck radiator cap, knowing what could be responsible for the issue can help in fixing the problem.

If you’re having a tough time removing your vehicle’s stuck radiator cap, kindly pay attention to some potential causes of the problem below.

Pressure build-up

The radiator cap is designed to maintain a particular level of pressure within a vehicle’s cooling system, which can help protect the engine from overheating. 

However, if excessive pressure builds up, it can automatically make removing the radiator cap difficult. This most likely occurs if the vehicle’s engine has been running and the cooling system is still hot.

Improper installation 

The radiator cap can also become stuck and difficult to remove if you failed to install radiator correctly the previous time you topped off the coolant or serviced the system. In other cases, cross-threading the cap during installation can also make it stuck.


Corrosion is almost inevitable for any metal component. Therefore, the metal components of your car’s cooling system can become rusted or corroded over time. Corrosion can affect the radiator cap and make it become stuck. This usually occurs in older vehicles.


A car’s radiator can also get stuck and become difficult to remove if debris or dirt gets into the radiator cap and accumulates around that area. Meanwhile, lack of proper maintenance can give access to dirt and other contaminants to accumulate in the radiator cap area.

Damaged radiator cap

Another reason you may struggle with removing your car’s radiator cap is if the component is faulty or damaged. A broken radiator cap can be tough to remove in some cases. Due to damaged radiator you may also notice bubbles in radiator.

Generally, the radiator cap’s rubber gasket can become cracked or worn over time, thereby preventing the cap from sealing properly. The cap may eventually become stuck when this occurs, making it hard to remove.

How to Open Stuck Radiator Cap without Damage

If your vehicle’s radiator cap won’t come off, please do not force it. Doing so can cause severe damage to the component or the radiator. Rather, figure out a better way to remove it with utmost care.

So, if you’re wondering how to remove a radiator cap, here are a few tips to help you open a stuck radiator cap without damage.

Opening Stuck Radiator Cap YouTube

Prioritize safety

Before making any attempt to open your car’s stuck radiator cap, ensure to turn off the engine and let it cool down completely. Otherwise, going on with the radiator cap removal can burn you when the engine is still hot.

Use protective equipment

Again, before removing a radiator cap, ensure to wear a pair of hand gloves or use a piece of rag to grip the radiator cap. This will help to prevent your hands from slipping off and getting burnt by the hot coolant in the radiator.

Turn the radiator cap with your hands.

If the radiator cap is not very tight, try to turn it with your hands. Ensure to use both hands while gripping the cap firmly.

Then, carefully twist it counterclockwise to enable it to loosen a bit. If the cap is still too hot, you may want to use a pair of pliers with rubber jaws.

Use a lubricant 

Now, if the radiator cap is still stuck in place, using a lubricant such as WD-40 or a penetrating oil can help to loosen it.

So, apply some lubricant around the radiator cap’s base and let the fluid sit for some minutes. After that, you can attempt to turn the cap once again.

Use a strap wrench

Again, if the radiator cap proves to be stubborn and won’t budge, it may be time to use a strap wrench. This tool can help to grip cylindrical objects in order to provide more leverage than when using your hands or pliers.

So, position the strap wrench around the radiator cap and tighten it until it gets a good grip on the cap. Then, carefully turn the strap wrench in a counterclockwise direction to loosen the radiator cap.

Finally, remember to be patient and gentle as you attempt to open the stuck radiator cap. If your coolant reservoir cap won’t come off, the first attempt could get the job done, while you may need to move from option one to two or three before seeing any result.


If your radiator cap won’t come off even after carefully following the recommendations in this article, you may need to contact an expert auto technician to get the component off as soon as possible.

Remember, the goal is to remove the radiator cap without causing any damage. Therefore, don’t try to forcefully remove the cap by all means so that you don’t damage it and incur unnecessary expenses for the replacement.


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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