Dealing with a Missing Coolant Reservoir Cap!

Driving without a reservoir cap is risky business, so what do you do if the coolant reservoir cap is missing? To jump straight to the point, driving without a coolant cap will cause the engine’s coolant to boil and evaporate. If a car engine becomes too hot, it will spit coolant out of the reservoir, leading to overheating. In the worst case, this can result in head gasket failure.

Now that the worst parts are out let’s understand what a coolant reservoir cap is and what it does. Then, you can decide whether you risk driving with the coolant reservoir cap missing or seek replacement first.

What Is a Coolant Reservoir Cap?

A reservoir cap or radiator pressure cap is a mechanical tool that regulates the pressure of the heated coolant in the radiator. The radiator is a reservoir that holds the heated coolant, cooling it as air passes through the radiator core before returning the cold coolant to the engine.

What does a coolant reservoir cap do?

The radiator cap or coolant reservoir cap does not just function as the cover for the bottle, preventing coolant from escaping. It also contains the pressure built up in the system. As the temperature of the coolant rises, it soon begins to boil after some time and turns into gas molecules or vapor. 

In water, the boiling point of the liquid is about 212 degrees Fahrenheit (100 degrees Celsius), which also rises when under pressure. The radiator cap helps to boost the coolant’s boiling point by approximately 40 degrees Fahrenheit, which ensures that no “boil off” of coolants occurs, meaning the coolant remains a stable liquid.

How the Coolant Reservoir Caps Functions in a Pressurized System

One point to remember is that liquid expands as it heats up. When coolant is heated up, it broadens or gains in volume, giving rise to additional pressure in the cooling system. In a pressurized cooling system once the cooling system attains the rated force of the cap, the pressure spring contracts and permits the moving of the coolant into the reservoir tank. This cycle enables the coolant to expand while maintaining the proper operating pressure of the cooling system. 

How the Coolant Reservoir Cap Functions in a Non-pressurized System

When the engine is turned off, the pressure inside the car starts to decrease. At this point in a non-pressurized cooling system, the coolant reservoir cap uses a vacuum valve to allow the coolant to flow from the reservoir tank back to the radiator. This process happens during the cooling period. As the coolant temperature decreases, the coolant starts to contract, creating a vacuum. The vacuum valve then opens up and pulls the coolant back into the radiator from the reservoir tank. As a result, the coolant level returns to its initial level.

coolant cap missing

What Happens if You Leave the Coolant Reservoir Cap Off?

Leaving your coolant reservoir cap off is not advisable, whether in a pressurized or non-pressurized system. Both types of systems mean you have different things going on when driving without the cap:

  • In a pressurized system – If the Coolant Reservoir Cap is left off, it can eventually lead to overheating. As you drive the car, it will run out of coolant due to the constant spilling (going over bumps and turning) from the coolant reservoir. The vehicle may be unable to adequately regulate the heat generated by the running engine due to insufficient coolant. 
  • In a non-pressurized system – As soon as the car’s engine is switched off, the coolant contracts and begins to chill. If the coolant reservoir cap is left off, the system cannot prevent an influx of air. When air gets in, it results in poor engine cooling. Air getting into the system is the primary factor leading to cooling system component corrosion and overheating when you next start the engine.

Regardless of which coolant system you have in your car, the long-term problems of driving without a coolant cap include the following: 

  • Loss of coolant – Without the cap, the coolant can evaporate, causing a loss of coolant from the reservoir. This can cause the engine to overheat and damage the engine. 
  • Contamination – The coolant can get contaminated with dirt, debris, and other contaminants if the cap is absent. This can cause damage to the water pump and radiator.
  • Air ingress – The coolant system can develop air pockets if the coolant reservoir cap is missing. This can cause the engine to overheat, leading to engine damage.

As a side note, the coolant varieties and improper coolant combinations can likewise result in the car overheating.

Can You Drive With a Coolant Reservoir Cap Missing?

The answer to that question is yes. Though I not recommended it, you can drive without a coolant reservoir cap present. The coolant reservoir cap, or the radiator cap, does not prevent your car from starting and running. 

However, suppose you were to drive around in your vehicle without the coolant reservoir cap. In that case, there is the probability that when the engine heats, as the temperature rises, the coolant will steam and overflow, causing coolant to leak from the reservoir. If a lot of the coolant is wasted, your car engine may suffer overheating.

So, avoiding any difficulties that might further affect your car’s engine is best by ensuring that the coolant cap is replaced before driving. This might not help you if it’s already missing now.

What to Use if You Lost the Coolant Reservoir Cap?

Suppose you’re looking for an alternative to replace a lost coolant reservoir cap use a PVC pipe, foil, and duct tape on an older vehicle. Attaching the pipe covered in foil onto the reservoir opening with duct tape would be a temporary solution to get you to an auto parts shop to seek a replacement. However, as most modern vehicles use a pressurized cooling system, it would be difficult to keep the coolant under pressure with this method.

Unfortunately, replacing your missing coolant reservoir cap is the only course of action for a modern vehicle. It is a risk, but you may need to drive without a coolant cap temporarily to get to an auto parts shop for a replacement. In this case, as long as we only talk a couple of kilometers, It should be ok; the coolant will need topping back up to the correct level before you put on a new cap. But be warned, you do this at your own risk; there are no guarantees, even driving just a few kilometers.

Frequently Asked Questions 

Are Coolant Reservoir Caps Universal?

Coolant reservoir caps are not universal; different vehicles require different sizes and types of caps to fit their specific reservoirs. Temporarily, using a coolant cap from the same vehicle brand will often work, i.e., it will fit as it has the same thread but may have the incorrect pressure rating. Using an incorrect cap can lead to leaks and other cooling system issues.

Can A Missing Coolant Reservoir Cap Cause Head Gasket Failure? 

It’s unlikely that a missing coolant reservoir cap alone would cause head gasket failure. However, it could cause a loss of coolant, which could cause the engine to overheat and lead to head gasket failure.


As you can see in this article, a coolant reservoir cap is a very vital piece in a car cooling system. While you can drive your vehicle temporarily without the coolant reservoir cap, I do not advise it, as it might cause the vehicle to overheat and even cause damage to the engine. It is essential to keep the coolant reservoir cap in good shape by replacing it if damaged or missing immediately.

My name is Tom although my friends call me Tommy. Messing around with cars and bikes has always been a hobby of mine even from a young age. So I made it my day job 17 years ago. I am a fully qualified mechanic as you would expect. I've worked in all different areas of the motor trade, valeting, panel beating, engine repairs, I'm sure you get the idea. I enjoy sharing my wealth of knowledge and experience with others, which is the reason I spend a lot of time here writing for this website.

2 thoughts on “Dealing with a Missing Coolant Reservoir Cap!”

    • Thanks for your comment.
      If you are unable to find a new cap with a seal for your car, consider checking with a nearby car breaker yard for a second hand one. A lot of car brands have interchangeable parts. For example, if you own a Ford, you may find a Ford coolant cap from a different model that fits yours.

      Good luck


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