How Long to Wait to Open Coolant Cap?

How Long to Wait to Open Coolant Cap

Part of a vehicle’s engine is the pressurized cooling system that prevents it from overheating by removing excess heat from the engine.

The cooling system as a coolant cap also known as the radiator or pressure cap helps to keep coolant fluid in even when it is at a boiling point

The coolant cap serves as an escape route for pressurized water in the system. The pressure builds up to a boiling point when the engine is running, at this point it will be unsafe to open the cooling cap.

Since it is unsafe to open the cooling cap when it is hot, how long to wait to open coolant cap?

How long you should wait before opening a coolant cap depends on the time the car takes to cool down. But on average, you can wait at least 1 hour to 2 hours before opening the coolant cap to avoid burning your face or hand with water or coolant.

Can I Open the Coolant Cap?

Absolutely, a coolant cap can be opened but under no condition should it be opened while hot.

Opening the coolant cap when hot is a recipe for disaster because at this point the engine is very hot or overheating which means the pressure inside is at its highest.

Opening at this point will result in you having hot steam in your face and probably some parts of your body leading to burns and injury.

In order to prevent this and be on the safe side, it is advisable to allow the system to cool down before you open it. When the system eventually cools down, do not open the cap with your bare hand.

How Long to Wait to Open Coolant Cap?

Since it is said that a coolant cap shouldn’t be opened when hot, how long does one have to wait before opening it?

How long you have to wait will depend on the length of time it takes the engine to cool down. The time taken can be short or long depending on some of the following; the weather condition, how hot the engine is at the time, and the level of coolant in the engine.

An engine that has been running all day will take a longer time to cool than one that was only taken out for a spin.

Irrespective of the aforementioned factors, it is usually safe to open a coolant cap within an hour or 2 hours. Waiting this long will give the engine ample time to cool down.

There are ways to know if it is safe to open a coolant cap, one of which is by feeling the cap top with a soft cloth or gloves. If the pressure is still high the cap will be very hot but it reduces as it cools down.

Why Should I Wait Before Opening Coolant Cap?

You must have heard people over time saying, you should wait till the engine cools down before opening the coolant cap, but you don’t really understand why this is.

An engine coolant when at its hottest can get as high as 260֯F, at this temperature it is very dangerous to open the coolant cap especially if the engine has been running for a long period of time.

Opening the cap at this temperature causes it to blow off bringing steam and steam of hot water with it. The force with which it comes out leads to the spray of hot steam on anyone and everything around it which can result in burns and injuries.

It is also beneficial to the engine when you allow cooling before opening the cap.

Pros of Allowing the Cool Down Before Opening Coolant Cap

There are advantages of allowing a cool down before opening a cooling cap, some of which are as follows;

  • It Prevents Internal Combustion: opening a hot coolant cap allows air inside which when mixed with the pressurized fluid can cause internal combustion inside the engine over time. An advantage of allowing a cool down is you are preventing combustion from happening.
  • Burns and Injuries are Avoided: Jets of hot steam sprays out when you open a hot coolant cap which can result in burns and injuries, by leaving it to cool, you are doing yourself a lot of good.

Can I Add Coolant to Reservoir When Engine is Hot?

No, don’t do it. Adding coolant to the reservoir when the engine is hot isn’t always a good idea, it is best to wait till it’s cold before adding.

There is a change in temperature when you add coolant also known as “antifreeze” to the reservoir while it’s hot. This change damages the engine by forming cracks around the engine block.

Waiting to add coolant to the reservoir is in your best interest as you get to have a functional engine for much longer and permanent scarring from burns can be prevented.

What Happens if you Add Coolant to Hot Engine?

It is never a good idea to add coolant to a hot engine, this will only lead to more overheating which causes cracks to appear in the engine block. A cracked engine block renders the engine useless.

How Long Should I Wait to Start my Car After Adding Coolant?

For the safety of all concerned, it is advisable to wait at least 15minutes to start the car after you must have added coolant to the reservoir. Also, you should also let the engine run for 5 minutes after adding the coolant before driving it.

It takes some time (not too long) for the coolant to cool down the engine by returning the temperature back to normal.

When you give the engine enough time to cool before starting the car, it takes a longer time for it to overheat all things being equal i.e. if nothing else is wrong with the engine or cooling system.


The coolant cap is often time overlooked as a part with minor function in the engine but this is not so, without the cap in place a lot of things will go wrong in the engine and car as a whole.

When you are not sure how long you should wait for the car to cool down, you can start by touching the coolant cap, if it’s still hot inside the cap will also be hot. The hotness of the cap reduces as the temperature inside normalizes.

This should give you an idea of how much longer you’d have to wait. When feeling for the temperature, it is better you do not use your bare hand.