Coolant Reservoir Cap Missing [What To Do?]

Do you know that the primary cause of car overheating is the leakage of coolant?

Coolant often leaks due to the missing of a coolant reservoir cap. This might lead to overheating. As a result, severe damage occurs to your car engine.

The challenges faced by the car can increase the risk of damage to its engine. It is either due to the coolant reservoir cap missing or a bad coolant reservoir. A coolant is responsible for lowering the temperature of a machine. 

Before diving into the tips and tricks to fix this problem, let’s first try to understand the working of a coolant reservoir. 

How Does A Coolant Reservoir Work?

You might have encountered the coolant reservoir multiple times while repairing your car. Therefore, understanding the mechanism and parts of it is essential.  

A coolant reservoir is a container mounted on a car’s engine. This reservoir absorbs and expels a coolant in order to cool down the engine. Apart from this, it also warms up the engine when the temperature is too low. Therefore, one can call it a storage system for a coolant. 

When the car engine works, there is a lot of heat produced. The temperature may rise beyond a limit if no coolant is present in the car. That’s because the engine undergoes a combustion cycle. The result of this process is excessive heat that can bend the cylinders. 

Parts Of A Cooling System

A coolant reservoir

A coolant reservoir stores coolant in your vehicle. This reservoir is made of high-quality plastic. It is located near the radiator and on the right-hand side of the engine. 


The coolant reservoir is connected to pipes that carry coolant liquid. The liquid flows around the engine to lower its temperature. It absorbs heat and keeps the engine cool.  


A cooling water pump of a car supplies heat exchangers and cooling water. It pushes the coolant into the engine and radiator. 

A radiator

A radiator is a device that removes excess heat from the car’s engine. The coolant passes from the radiator to the engine and back to the radiator. 

What Will Happen To Your Car If The Coolant Reservoir Cap Is Missing?

The real problem with a coolant reservoir cap is that without it, the coolant reservoir does not work efficiently. That’s because the primary function of this cap is to avoid the leakage of the coolant liquid from the system.

When the cap isn’t in its place, the heated coolant will overflow from the radiator tubes. 

Additionally, the coolant is at higher risk of coming in contact with dirt and debris.

Along with this, the cap doesn’t let the coolant spill out from the reservoir when you are driving on a bumpy road. So, the cap can avoid the overflowing of the liquid. 

Another problem that may occur when the cap is lost is that the tank’s coolant evaporates faster. In such a situation, the quantity of the coolant will lower within a certain period of time. Finally, it will cause overheating. 

The reservoir cap plays a crucial role in the coolant boiling. It exerts pressure on the cooling system to lower the boiling process.

So, the coolant will overflow outside the tank when the reservoir has no cap. Also, the radiator will release coolant into the reservoir system.

Steps To Follow If Coolant Reservoir Cap Is Missing

The solution to this problem entirely depends on what type of cooling system you have. There are non-pressurized and pressurized cooling systems. Now, you must be still wondering what to do if your coolant reservoir cap is missing. No biggies. Here are a few tips to fix this issue. 

Steps To Follow If You Have A Non-Pressurized Coolant System

A non-pressurized coolant system is not directly attached to a radiator. That’s why you won’t face significant challenges while solving this issue. This is a win-win situation as the missing cap would not affect the performance of your car. 

However, dirt and debris enter the reservoir system. In addition, the coolant may also spill out.

You can stop the leakage by dropping two eggs in the cooling system. It will prevent leakage. But if the leakage did not stop, try adding a few more eggs into it. 

You can drive your vehicle without the cap but make sure you are not out of the coolant. That’s because the engine will heat up, and you might have to replace it. So, block the hole with an old rag to be on the safer side. 

Later, you can purchase a new cap from a store. The cost of this cap ranges between $10 to $15. 

Steps To Follow If You Have A Pressurized Coolant System

The new car models are coming with a pressurized coolant system. These systems are connected to the radiator.

As these systems have a higher boiling point than a non-pressurized one, you do not need to worry about the damage your cooling system may have. 

This system can hold high pressure without causing steaming and evaporation of the car coolant.

As a result, there is less pressure built up in the reservoir without a cap. However, if you don’t have the cap on the pressurized system, don’t drive the car for an extended time. Otherwise, it will increase the leakage by 10X. 

The only thing that you can do is buy a new cap from the local store. Make sure you know your car model beforehand. 

Bottom Line

The cooling system of a car shows issues now and then. Therefore, you can keep the above guide handy.

We have tried to cover the significant problems that you might face along with some tips. 

We have simplified the situation.

So, now you don’t need to stress the missing coolant reservoir cap. Just figure out the best approach for your vehicle’s problems.