Can You Drive Without a Coolant Reservoir Cap?

The coolant reservoir cap is an important component of a car’s cooling system. It is designed to keep the right pressure in the coolant system which is between 12 and 15 PSI. But what happens when you try to drive a car without a coolant reservoir cap? Read on to find out.

Key Takeaway

  • Technically, you can drive without a coolant reservoir cap but keep in mind that over time, coolant will escape in a form of vapor.
  • Coolant caps are not universal and each vehicle manufacturer uses different materials, sizes, and shapes of caps.
  • A missing coolant reservoir cap could cause the engine to overheat because coolant will continually evaporate or spill which will lead to not enough coolant in the cooling system.

Can You Drive Without a Coolant Reservoir Cap?

Can You Drive Without a Coolant Reservoir Cap

Yes, you can drive without a coolant reservoir cap. However, when driving without a coolant reservoir cap, the coolant will escape the coolant reservoir in a form of vapor without a trace, or it can spill out while driving on a bumpy road. Without the required amount of coolant in the cooling system, the engine will quickly overheat.

A coolant reservoir is a closed system like the rest of the cooling system. The coolant reservoir serves to cool down the hot coolant and to reduce the pressure by releasing it through the reservoir cap. The coolant reservoir cap is designed to hold pressure up to about 15 PSI. Anything above 15 PSI and the cap will open up and allow the extra pressure to escape.

When driving without a coolant reservoir cap, the pressure within the coolant system will be disrupted. The flow of the coolant around the engine relies on the water pump. However, it also relies on pressure to keep it circulating around these important engine components. If the reservoir cap is missing, the cooling system won’t be able to keep up the required pressure.

This means that the circulation of the coolant will be reduced. The engine temperature will start to slowly increase which is not a good thing. The water pump will of course try and maintain the flow. But, this will only put more strain on the water pump which can result in premature wear.

To summarize, you can drive without a coolant reservoir cap, but it is not advisable. You will experience a loss of coolant from spillage and evaporation and reduced coolant circulation due to a lack of pressure in the cooling system.

Are Coolant Reservoir Caps Universal?

No, coolant reservoir caps are not universal. In fact, it is safe to say that every car brand has its own type of coolant reservoir cap. There are many differences between coolant reservoir caps like features, shapes, sizes, and materials. Choosing the correct coolant reservoir cap for your vehicle is essential.

If you notice that your coolant cap is failing, it is important to take action. The cap is commonly looked at as nothing serious, but that is far from the truth. A damaged coolant reservoir cap can cause a coolant to leak, the engine to overheat, or burst radiator hoses.

The cooling system is pressurized. This makes the coolant reservoir to be pressurized as well. The coolant reservoir cap holds pressure between 12 and 15 PSI before it opens up and releases that pressure into the atmosphere. However, a faulty pressure cap can be constantly open allowing pressure to escape, or it can be closed and allow pressure to build up and burst some radiator hoses.

Each vehicle has a different size coolant reservoir. This means that the coolant reservoir caps are not universal. You have to read the owner’s manual to find out what is the size, feature, and shape of your cap and get the exact same one.

How Do You Replace a Coolant Reservoir Cap?

If you are looking into replacing your coolant reservoir cap because it is broken, missing, or can’t hold pressure anymore, there are a few things that you should be aware of.

1. Order the exact same coolant reservoir cap

Can You Drive Without a Coolant Reservoir Cap

When replacing a coolant reservoir cap, you have to make sure that you order the same type of cap. The coolant reservoir caps have the same purpose, but they do not share the same design. This is why it is important to buy a new coolant reservoir cap that is specifically made for your vehicle.

If you are going to buy a new cap from a store, always take the old one with you. This way you can check for differences on the spot. If you decide to order one online, make sure that you enter the make, model, and year of your vehicle and get the exact same cap.

2. Make sure the vehicle is cold

The coolant reservoir serves as a getaway for hot coolant. This means that when the engine is running, there is a lot of hot coolant in the coolant reservoir. This is why it’s important to always make sure that the engine is cold when trying to replace the reservoir cap.

If you decide to open the coolant reservoir cap when the engine is hot, the coolant may spray out because there are between 12 and 15 PSI in the reservoir. A great way to tell if your engine is hot or cold is by looking at the temperature gauge.

3. Remove the old cap

Can You Drive Without a Coolant Reservoir Cap

Removing the old coolant reservoir cap can be quite hard, depending on how damaged the old cap was. Try to remove it as safely as possible to avoid getting some pieces into the coolant reservoir. After removing the old cap, inspect the threads on the coolant reservoir. As you can see, the coolant reservoir is plastic and it is constantly working with hot coolant. So, damages to the reservoir, the cap, or the threads are nothing new.

4. Fill up the coolant reservoir

Can You Drive Without a Coolant Reservoir Cap

While you are at it, make sure to inspect the level of the coolant. There are markings on the side of the reservoir that suggest where is the low mark and where is the full mark. Since you were dealing with a bad coolant reservoir cap, there is a good chance that some of the coolants evaporated or leaked out of the reservoir. So, grab a 50-50 mix of antifreeze and water and refill the reservoir to the correct level.

5. Install the new cap

After you refill the coolant in the reservoir, it is time to install the new cap. Make sure to tighten the cap as hard as you can. Then, start the engine and wait for it to reach the optimal working temperature. When the engine reaches the proper temperature, the coolant will circulate, get hot, and then will go to the coolant reservoir to cool off. This is when you should be out there inspecting the new coolant reservoir cap. Make sure that it is able to hold pressure and release it when needed.

Can a Missing Coolant Reservoir Cap Cause Overheating?

Yes. A missing coolant reservoir cap can cause overheating because coolant can easily spill out of the reservoir while driving, the hot coolant can also evaporate from the coolant reservoir, and most importantly, without the coolant reservoir cap, there will be not enough pressure in the cooling system which will result in weakened coolant circulation and engine overheating.

Engines produce intense heat because sparks are igniting an air/fuel mixture in the combustion chamber and because the engine components are moving against each other continuously. Most vehicle engines operate in temperatures between 195 and 220 degrees Fahrenheit.

To keep the engine components from overheating, the cooling system is constantly circulating coolant through the engine. The cooling cycle begins with the engine being started. When you start the engine, the water pump is started as well because it is driven by the timing belt. The water pump pulls coolant from the bottom of the radiator and starts to circulate it through various passages inside the cylinder head and engine block.

When the coolant that has been circulating around the engine components gets hot, it expands and creates pressure. Having a hot coolant and excess pressure is not acceptable for the cooling system because it can lead to damage. This is why the hot coolant goes to the coolant reservoir to cool down. Also, the extra pressure gets released into the atmosphere by the coolant reservoir cap when it gets above 15 PSI.

The coolant reservoir cap is there to prevent the hot coolant from spilling out or evaporating into the atmosphere. Also, the reservoir cap controls the pressure of the cooling system between 12 to 15 PSI. Without the coolant reservoir cap, your vehicle will overheat.

Is a Coolant Reservoir Cap Important?

The purpose of the coolant reservoir cap is to make sure there is constant pressure (12-15 PSI) and to prevent coolant from spilling out when it reaches the coolant reservoir. The coolant reservoir caps come in different shapes and sizes because each vehicle works with different pressure.

The coolant reservoir cap is important because it prevents air from entering the cooling system. If air gets into the cooling system through a faulty coolant reservoir cap, it can result in engine overheating, which leads to a blown head gasket and other damage.

On top of the reservoir cap, you may find two types of measurements: psi and kPa. They both stand for pressure, but it is just different measurement systems (imperial and metric). Most reservoir caps are rated from 4 PSI (30 kPa) to 30 PSI (205 kPa). The higher rating is usually found in performance vehicles.

When choosing the right reservoir cap for your vehicle, you have to get the same one as before. The reason for this is that not all reservoir caps are the same. Each vehicle has a specific cooling requirement, thus, different cooling reservoirs.

Bob Semana

Hi there, I am a Mechanical Engineer that specializes in AC, Alternators, Batteries, Cooling systems, and Drive Train issues.

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