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Coolant Reservoir Cap Missing (What to Do)

Updated: Jan 13

A coolant reservoir cap is an essential component of a car's cooling system. It plays a crucial role in maintaining the pressure and level of the coolant in the system. The coolant reservoir cap is a small but vital part of the car that is often overlooked.


It is important to understand the implications of a missing coolant reservoir cap and what to do if you find your coolant cap is missing.

 

Table of Contents:


a yellow coolant reservoir cap

What Is a Coolant Reservoir?

A coolant reservoir, also known as a radiator overflow tank, is a component of a vehicle's cooling system. Its main function is to store excess coolant that expands when the engine becomes hot.


This excess coolant is then drawn back into the radiator as the engine cools down, maintaining the proper coolant level and preventing overheating.


The coolant reservoir is typically made of plastic and is located near the radiator, often in the front of the engine compartment. It is connected to the radiator by a hose and has a low and high level indicator, which alerts the driver when the coolant level is too low or too high.


What Happens When Missing a Coolant Reservoir Cap?

The coolant reservoir cap is responsible for maintaining the pressure in the coolant system. It's also known as a coolant overflow tank or expansion tank cap.


When the cap is missing, coolant can escape from the system, leading to a decrease in pressure. This can cause a number of problems, depending on the type of coolant system in your car.


Pressurised Cooling System

In a pressurised coolant system, the coolant is kept at a higher pressure than the atmosphere. This helps to raise the boiling point of the coolant, allowing it to absorb more heat from the engine.


When the coolant tank cap is missing, the pressure in the system can drop, causing the coolant to boil at a lower temperature. This can lead to engine overheating and damage.


Non-Pressurised Cooling System

In a non-pressurised coolant system, the coolant is not kept at a higher pressure than the atmosphere. When the coolant reservoir cap is missing, coolant can escape from the system, leading to a decrease in the coolant level.


This can cause the engine to overheat as the coolant is not able to absorb as much heat from the engine.


Can You Drive Without a Coolant Reservoir Cap?

It is not recommended to drive without a coolant reservoir cap as it can lead to engine damage and overheating. A missing coolant reservoir cap can lead to a decrease in the coolant level.


Is a Coolant Cap Important?

A coolant reservoir cap (and radiator cap) is important as it helps to maintain the pressure (if the cooling system is a pressurised one) and level of the coolant in the system. It is where coolant can be added, without the coolant filler cap debris can enter the coolant system.


The coolant reservoir cap also stops coolant from spilling out and leaking.


What to Do if You Lose Your Coolant Cap?

If you lose your coolant cap, it is important to replace it as soon as possible. A missing coolant reservoir cap can cause the engine to overheat and lead to potential engine damage.


You can also try some of the quick fixes detailed below so you can get to where you need to go.


Missing Coolant Reservoir Cap Quick Fix

If you lose your coolant cap and are unable to replace it immediately, a quick fix is to use a cloth and duct tape to cover the opening of the coolant reservoir. This will help to prevent coolant from escaping from the system, but it is not a permanent solution.


Temporary Fix for Non-Pressurised Cooling Systems

If you have a non-pressurised coolant system, there are a few temporary fixes that you can use to prevent coolant from escaping from the system.


  • Use a cloth and duct tape to cover the opening of the coolant reservoir.

  • Use a heavy-duty rubber glove and zip ties to cover the opening of the coolant reservoir.


Temporary Fix for Pressurised Cooling Systems

A pressurised coolant system works differently from a non-pressurised cooling system, it is important to use a more secure temporary fix to prevent coolant from escaping from the system.


You can use the same methods to temporarily fix a missing coolant reservoir cap as you would for a non-pressurised cooling system. However, the coolant system won't work properly and could result in engine damage or overheating.


Only drive for short periods of time and keep an eye on the coolant temperature.


Conclusion

A coolant reservoir cap is an essential component of a car's coolant system. It plays a crucial role in maintaining the pressure and level of the coolant in the system, if it's gone it can't do maintain pressure and keep debris out of the coolant system and tank.


If you find yourself without a coolant reservoir cap, it is important to replace it as soon as possible to prevent engine damage and overheating.


In the meantime, there are several temporary fixes that can be used to prevent engine coolant from escaping from the system if you have a missing coolant reservoir cap, but it is important to use the appropriate fix for your type of coolant system.