Coolant Reservoir Cap Missing? [Avoid Engine Overheating]

You’re cruising down the highway on a warm summer day, the wind whipping through your hair and the music blasting. Suddenly, you notice the temperature gauge creeping into the danger zone. Your heart races as you pull off to the side of the road and realize that your coolant reservoir cap is missing. It might seem like a trivial part, but it actually plays a crucial role in keeping your car’s engine cooling system functioning properly. In this guide, we’re going to take a closer look at what a coolant reservoir cap is, why it’s important, and what to do if you find yourself with a missing one.

So, what’s the scenario if coolant reservoir cap is missing? A coolant reservoir cap is a small cap that covers the top of the coolant reservoir in your car, and it’s responsible for maintaining the pressure in the cooling system and preventing coolant from boiling over. If the coolant reservoir cap is missing, then the coolant will boil and evaporate at a much faster rate than normal. As a result, the coolant will splash out of the engine, and the engine won’t be able to stay at its optimal operating temperature and will eventually overheat. You should keep eye at the temperature gauge as overheating can cause major damage to the engine and other components of the vehicle. So, if coolant reservoir cap is missing, you can put some tape on the coolant overflow reservoir and drive to nearest shop or your home to get a new coolant reservoir cap. You can fix seran wrap with a rubber band over coolant overflow tank if cap is missing.

What Is Coolant Reservoir and Why Is It Important?

Coolant reservoir is an important component of any car. It is a white color plastic container located in the engine bay of a vehicle that holds the coolant fluid for the cooling system. marked with a “minimum” and “maximum” line, so you can tell when the reservoir needs to be topped up.

The coolant reservoir, often referred to as the coolant overflow tank, is responsible for storing excess coolant fluid that the radiator has pushed out when it reaches a certain level of pressure. Coolant reservoir is located near the radiator, and is connected via a hose to the radiator cap.

The radiator cap is designed to let out a certain amount of pressure, but if the pressure builds up too high, it will push some of the coolant out of the radiator and into the reservoir.

how coolant reservoir is linked to the radiator

In the above picture, I have made a simple illustration of how the radiator cap is connected to the coolant reservoir via a hose. I have two cars i.e. Toyota Vitz and Hyundai Elantra. I have attached pictures of coolant reservoirs in both vehicles:

hose connecting coolant reservoir with the radiator cap

In both vehicles, you can see that a hose goes from a radiator cap to the bottom of coolant reservoir aka overflow tank. However, you will also find some vehicles in which that hose from radiator is connected to the cap of coolant reservoir. 

Such vehicles have coolant reservoir caps like this:

coolant reservoir cap

The coolant reservoir is important because it helps to keep the coolant in the system during times of high pressureWhen a cooling system is pressurized, the coolant expands and flows to the coolant overflow reservoir. When the pressure in the cooling system settles down, the coolant reservoir allows coolant to be pushed back into the radiator and the engine.

if there is no coolant reservoir and the hose from the radiator cap is just open to the atmosphere, it will suck air back into the system instead of coolant. If air is introduced into the cooling system, it causes overheating and a head gasket.

Furthermore, with a white-colored plastic coolant overflow tank, you can safely monitor the color of the coolant and check its level. If there is no coolant overflow tank, you will have to regularly remove the radiator cap to see level of coolant and visually inspect its color to check if it’s dirty or not.

Furthermore, removing the radiator cap is quite unsafe if the engine is hot as it will cause hot coolant to splash out and burn your skin. An inexperienced person might not properly tighten the radiator cap as it requires a technique, unlike the coolant reservoir cap.

What Is The Role Of A Coolant Reservoir Cap?

The purpose of a coolant reservoir cap is as follows:

  • To keep the coolant in the system
  • To prevent debris from entering the system
  • To maintain proper pressure in the system
  • To prevent air from entering the cooling system

The main purpose of the coolant reservoir cap is to keep the coolant contained and prevent it from leaking. This helps to ensure that the coolant will stay within the system and not be lost, which can cause the engine to overheat or even to seize up. In addition, the cap helps to ensure that the pressure in the system is maintained, which helps to keep the cooling system working as efficiently as possible.

The coolant reservoir cap is also important for safety reasons. It prevents dirt and debris from entering the reservoir, which can cause contaminants to enter the system and damage the engine. In addition, the cap prevents the coolant from boiling over, which can cause unsafe amounts of steam to be released from the system

Basically, when the engine’s temperature increases, the coolant heats up and expands in the reservoir. 

Now, there is a basic Physics principle that when the pressure above the liquid surface increases, the boiling point of a liquid also increases. This is the reason that in a pressure cooker, food cooks faster as the water takes time to boil due to an increase in pressure.

Let’s apply this same principle to the coolant reservoir. When the reservoir cap is put on it, it makes a sealed container of coolant in which the coolant will expand but it will not start boiling and evaporating at a faster rate.

If the coolant reservoir cap is missing, the pressure above the surface of the coolant in the reservoir decreases as it is exposed to the pressure of surrounding air. As a result, the coolant will start boiling and evaporating at a faster rate.

Furthermore, due to boiling and the formation of bubbles, the coolant will start splashing out of the reservoir, and your engine will run out of coolant much earlier.

Lastly, if the coolant reservoir cap is missing, air will suck into the cooling system, and form air bubbles in the coolant. Due to this, the coolant will not circulate properly in the engine, and cause overheating.

Coolant Reservoir Cap vs Radiator Cap

radiator cap and coolant reservoir cap difference

A coolant reservoir cap is a simple cap that looks similar to a plastic bottle’s cap with internal threads. On the other hand, the radiator cap is a pressure relief valve designed to keep the pressure in the radiator from getting too high. It’s usually located on the uppermost part of the radiator and consists of a spring-loaded valve. The coolant reservoir cap is made of plastic while the radiator cap is made of steel.

The radiator cap has a pressure rating, which is usually stamped onto it. This indicates the maximum pressure the cap can handle before the valve opens, releasing pressure and allowing coolant to escape the radiator and flow to the coolant reservoir. On the other hand, the coolant cap does not have a specific pressure rating as its purpose is not to maintain pressure. The purpose of the coolant cap is just to seal the reservoir and prevent the coolant from splashing out.

Radiator cap is more like a mechanical part with a spring-loaded valve that opens when the pressure inside the radiator increases above a certain limit. The coolant reservoir is just a simple plastic cap like the one on a plastic Coca-Cola bottle.

Causes Of Coolant Reservoir Cap Missing

Here are the causes of the coolant reservoir cap missing:

  • Poor installation of cap: It is not uncommon for car owners to forget to put back the coolant reservoir cap after topping off the coolant. Human error can also include not securing the cap tightly enough, leading to it falling off while driving.
  • Wear and tear: Over time, the coolant reservoir cap may become worn out, making it easier to fall off or break. The aging or worn-out cap may also lose its sealing properties, leading to coolant leaks.
  • Vibrations: The coolant reservoir cap can come loose due to vibrations from the engine. Over time, vibration can cause the cap to loosen and eventually work its way off the reservoir tank. This is especially common in vehicles that drive on rough terrain or are used for off-roading.
  • Heat damage: Heat can cause the plastic material of the coolant reservoir cap to expand, which can cause the coolant reservoir cap to become loose and eventually work its way off. 
  • Coolant cap blows off: A blocked radiator can also cause the coolant reservoir cap to blow off. If the radiator is blocked, the coolant cannot flow freely and the pressure can build up in the system. If the coolant reservoir cap is too small for the opening, it may not be able to create a tight seal and could blow off under pressure. The same is true for a cap that’s been damaged or warped from exposure to heat or age. If the reservoir itself is cracked or damaged, it can create a pressure imbalance in the system and cause the cap to blow off. 

Driving Without Coolant Reservoir Cap

It’s not recommended to drive your car without a coolant reservoir cap for longer distances. It can cause engine overheating and safety risks in the long run. If the engine overheats, it can cause damage to other engine components, such as cylinder heads, head gaskets, and piston rings.

Overheating of the engine can cause a breakdown on the road, which can pose a safety risk to the driver and passengers. Furthermore, if you drive without a coolant reservoir cap, the coolant will continue to spill out of the reservoir and splash everywhere. 

While driving without a coolant reservoir cap, you should keep at temperature gauge of your vehicle. If it starts shifting to a high temperature, you should immediately stop your vehicle.

This is because the coolant reservoir is just an overflow tank. Your vehicle can still have the coolant in the radiator/engine. You will just have to keep an eye on the levels.

Lastly, avoid driving your vehicle with AC on if the coolant reservoir cap is missing, and try to drive at a constant speed. Don’t do hard acceleration with aggressive braking.

What Can I Use If I Lost My Coolant Reservoir Cap?

If you have lost your coolant reservoir cap, you can use a variety of makeshift items to seal off the reservoir. A piece of tape – duct tape or electrical tape both work – can be easily applied and will offer some protection. The tape will do an adequate job of keeping the coolant from spilling until you can get a new cap.

If you have a plastic wrap or seran wrap, you can use this as a temporary fix as well. Simply cover the opening with plastic wrap and use a rubber band or tie to secure it in place. Keep in mind that this is not a permanent solution and should only be used until a proper cap can be obtained.

Can I Use Any Coolant Reservoir Cap For My Car?

No, it’s essential to use the manufacturer’s recommended coolant reservoir cap to ensure that it fits and functions correctly. Using an incorrect coolant reservoir cap can cause the system to lose pressure, which can lead to overheating and engine damage. It is important to use the correct cap that is designed for your specific make and model of car.

Can A Missing Coolant Reservoir Cap Cause A Check Engine Light?

Yes, a missing coolant reservoir cap can trigger the check engine light, as it affects the engine’s performance and temperature. When the coolant reservoir cap is missing, there are a few things that can happen. The first is that the cooling system can lose pressure. This can cause the coolant to boil over, which can lead to the engine overheating. When the engine overheats, the internal components can become damaged, which can lead to a check engine light.

Another problem that can happen when the coolant reservoir cap is missing is that air can get into the system. This air can cause the coolant to become less effective at cooling the engine, and it can also cause the engine to run hotter than it should. Again, this can lead to a check engine light.