Function and Symptoms of A Bad Radiator Cap

You might know that your car’s radiator is responsible for keeping the engine from overheating. What is interesting and often overlooked is the importance of a small and seemingly unimportant component, the radiator cap. A bad radiator cap can render the whole cooling system ineffective and cause costly damage to the precious engine. Therefore, knowing the functions and symptoms of a bad radiator cap is a handy and straightforward DIY maintenance tip that you should pocket to maximize your engine’s performance and lifespan.

Coolant System In Car: Things You Should Know

Before we go into the details of the radiator cap, you must grasp the basics of a car’s cooling system. When it comes to a complex machine like your car, a small fault in a single component would affect the operation of the whole system and the symptoms may manifest through the other components in the same system.

The car’s radiator acts as a heat exchanger, transferring excess heat from the engine’s coolant fluid into the air. The radiator is composed of tubes that carry this coolant fluid, a protective cap that’s a pressure valve, and a tank on each side to catch the coolant fluid overflow. 

An interesting feature is that the tubes carrying the coolant liquid usually contain a turbulator, which agitates the fluid inside. This way, by creating turbulence inside the tubes, the coolant liquid gets mixed and thus is cooled evenly, not just the liquid that touches the sides of the tubes gets cooled. 

While the engine is running, the liquid from the coolant tubes enters into the engine heads and blocks. After collecting the heat from the engine, this liquid moves back into the radiator. There, the air entering through the grill brings down the temperature of the coolant

car radiator cap
A bad radiator cap can render the whole cooling system ineffective and cause costly damage to the precious engine. (Photo:

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Below are some of the major components of a car’s cooling system. There are more parts in the system, however, to understand the function of the radiator cap, this article will focus on the ones below:

Coolant fluid

This mixture in the coolant mainly consists of water and antifreeze solution with a 1:1 ratio.

Where the water extracts all the heat of the engine, the antifreeze solution, on the contrary, increases the boiling point of the solution. Thus, the latter helps in combating the corrosion and boiling of liquid in the system. 

A higher boiling point means the coolant can transfer more heat from the engine, as a boiling liquid is at the maximum temperature itself and thus cannot transfer any heat from the engine. 

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Coolant tubes

The main purpose of designing them in the cooling system is to build a bridge between the radiator and the engine. Responsible for circulating coolant, these are also known as bypass tubes.

Overflow tank

Located on each side of the coolant system, these small reservoirs collect the fluid thrown out during the high pressure in the tubes. By this action, they simply allow for the recycle of the coolant fluid, once the vacuum is generated in the coolant.

Radiator cap

This pressure cap functions by the spring-loaded valves. Its main use is to let the fluid out into the overflow tanks when the pressure inside the system increases. Similarly, on pressure-drop, it brings the liquid back from the tanks.

For your ease, if you don’t want to encounter a bad radiator cap, then always follow its maintenance tips.

How Does A Radiator Cap Work?

As the engine coolant heats, it expands, increasing the pressure inside the closed cooling system. The radiator cap controls this expansion and maintains a constant level of pressure inside the cooling system. The radiator cap also allows the engine coolant to contract and move back to the radiator without letting air enter the cooling system.

Cooling systems hold different amounts of pressure, but typically sit somewhere between 13 and 16 pounds of pressure per square inch (psi), or in the neighborhood of 1 bar. High-performance radiator caps range between 19 and 32 psi. This pressure needs to remain constant.

Too much pressure will exceed the manufacturer’s specifications for the cooling system and could cause cooling system component failure. Too little pressure could cause the coolant to boil off. With little or no liquid around to cool the vehicle, the engine is likely to overheat. 

Allows Coolant to Expand & Maintains Pressure Level

When the engine heats up, the coolant fluid overheats and expands, causing it to become highly pressurized.  When it enters the radiator, the pressure increases even more because it’s in an enclosed space. 

The radiator cap acts as a release valve set to open at the maximum pressure point – this is the radiator cap’s rated pressure. This is typically set at a density of 15 psi, otherwise at somewhere between 13 and 16 psi. 

When the pressure inside the cooling system reaches the radiator cap’s rated pressure, the cap’s spring-loaded valve is forced open, allowing heat to escape and excess coolant fluid to overflow into the coolant overflow tanks on either side of the radiator. 

This way, the radiator cap allows for the expansion of the coolant inside the cooling system. This is important because, without this feature, pressure would simply continue to build, causing sudden leaks or even explosions as the building pressure has no exit to relieve itself. 

Secondly, the radiator cap’s spring-loaded valve helps to maintain a specific pressure inside your cooling system. As atmospheric pressure rises, the boiling point of a fluid also rises. Boiling liquid isn’t effective at transferring heat as the liquid itself is overheated, so increasing the pressure inside the cooling system means the coolant can remove more heat before it begins to boil. 

Allows Coolant To Contract & Travel Back To The Radiator

The radiator cap also allows the coolant liquid to travel back to the radiator after it has completed its mission of cooling the running engine. As the engine cools down, the coolant contracts, creating a vacuum inside the engine’s cooling system. 

The radiator cap’s vacuum valve then opens and sucks the coolant from the coolant overflow tank back to the radiator, continuing its route through the cooling system. In other words, the vacuum valve of the radiator cap allows for the contraction of the coolant liquid inside the cooling system.

Symptoms of a Bad Radiator Cap

Replace a radiator cap
Replacing the cap on your radiator is an easy thing to do (Photo:

A bad radiator cap may not sound exactly dangerous, however, any such small deviation can greatly affect your engine’s performance and lifespan. Ignoring a faulty one can cause overheating problems and potential engine damage, especially during the hot summer months. If left for too long, a bad radiator cap might mean you risk being stranded on the road with a dysfunctional engine. 

Below are the most common symptoms of a bad radiator cap. Look out for the very first sign of an abnormality and have everything inspected as soon as you have the chance. 

Overheated engine

Leaky coolant or air in the cooling system can lead to an overheated engine. If you notice your engine starting to steam from getting too hot, don’t look under the hood unless you’ve turned the engine off. Then, let the engine cool for some time before popping the hood. 

That way, the engine can remain cool as you check it out. If there is coolant fluid near the radiator cap, there could be damage to the pressure cap. Check for that and replace as needed.

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Leaking coolant

If the radiator cap is stuck, pressure may build inside of the radiator, which could cause cooling system components to leak or burst.

If you notice coolant near the radiator or the radiator cap, then you have leaky coolant. Check to see if the radiator has holes in it or if the cap looks worn or damaged. If so, then replace the cap.

White streaks on radiator

When coolant leaks out of the radiator filler neck and dries, it often leaves white streaks behind. While you may not notice coolant leaking from under the radiator cap, look out for these white streaks. They may tell you the cap is leaking under pressure or intermittently. 

Overflowing reservoir

Coolant goes into the reservoir tank as it expands. The radiator cap releases the extra pressure by sending some coolant into the overflow tank.

If you have a bad radiator cap, the coolant could get released too quickly and cause the reservoir to overflow. While you’re in there, check to make sure your coolant overflow tank is working properly.

Radiator hose collapses

You may have a bad radiator cap if the radiator hose collapses. The vacuum won’t be released by the radiator cap properly and it will cause the radiator hose to collapse during the cooling down period.

If this happens, inspect the cap to see if there is any damage. If there is, replace it immediately.

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Radiator hose bursts

If the pressure in the cooling system is too high, you are likely to see one or more hoses start to spray coolant all over the engine bay. Most of the time the pressure isn’t high enough to rip the hose in half. You will often see a pinpoint leak that only sprays coolant when the car is warmed up.

A hose with a small hole may seal just fine when the car is cold. As you drive, pressure in the cooling system will build. The pressure will eventually be enough to force coolant through the tiny hole and your car will slowly lose coolant.

If this is the case, you’ll likely notice a leak on the ground or in the engine bay when you reach your destination, but not when you start driving. Check to see if your overflow reservoir is draining slowly throughout a few miles. This may give you a hint that this is the problem.

Air entering the cooling system

As mentioned above, an important role of the radiator cap is to allow the expansion and contraction of the coolant, or its movement through the cooling system, without letting any air enter the system. 

When your radiator cap does not seal properly, air could make its way inside the cooling system. This will cause air pockets to get inside of the heater core, thermostat, and radiator hoses. As a result, the engine will start to overheat because it cannot sustain a consistent temperature.

How to Check & Replace the Radiator Cap

Now, after knowing the symptoms of a bad radiator cap and its standard working, it must be easy to inspect the cap area and save your vehicle from any costly damage or repair. 

*Warning: Never open the radiator while it is hot! Allow the engine to cool completely before opening the radiator. The radiator is under high heat and pressure. Attempting to open a hot radiator will cause hot steam and coolant to spray out and is very likely to burn you.

Inspecting the cap

Before you get too deep into the diagnostics, double-check that the pressure indicated on the radiator cap matches the cooling system pressure specified by the manufacturer. You can find this information in a repair manual, factory service manual, or online.

Visually inspect that cap to make sure the spring moves freely and there is no debris or corrosion under the cap. 

Watch more to know how to replace a radiator cap in your car:

Replacing the cap

Replacing the cap on your radiator is an easy thing to do and may only require a few seconds of your time. 

Pay close attention to the pressure rating on your cap, and select one with the same pressure rating. Once you have a replacement and your engine is cool, simply unscrew the old cap, and install the new one by screwing it on. 

You can easily find a radiator cap for your vehicle for typically under $10. Some might be more expensive, but most will cost you less than $20.

Pressure testing the radiator

If you have a radiator pressure tester, then you can inspect the cooling system in your garage. This may help you find leaks or determine if the radiator cap has gone bad. You may also be able to rent or buy this tester from your local auto parts store or Amazon.

To begin, pop open the hood and find the radiator. It is usually located right in front of the vehicle’s engine. Remove the radiator cap by pushing it down first and then twisting it counterclockwise.

Take the cap adapter and screw it onto the pressure tester’s end. Keep screwing until you cannot screw it anymore. If you don’t know what the cap adapter looks like, check the label of the pressure tester’s packaging and see if it shows it there. You can also look it up on the internet as well.

After you’ve secured the cap adapter onto the pressure tester, screw the other end of the cap adapter onto the radiator filler neck until it is tight. The pressure tester should now be sealed against the radiator. If it is not, you may not be using the correct adapter for your specific radiator.

Using the pump handle, start pumping the tester until you reach the pressure indicated on your radiator cap. See if the gauge can store the pressure. If the pressure begins to fall and you’re sure you have a good seal against the radiator filler neck, you leak into the cooling system.

It is best to look for any external coolant leaks while the system is pressurized, as it will be easier to find this way. Any leaking components will need to be replaced.

When you’re done with your pressure test, unscrew the radiator cap adapter slowly so coolant doesn’t spill everywhere. It may be a good idea to have a pan or bucket handy to catch any that overflows. Top up any coolant that was lost, and clean up any coolant that has spilled onto the ground.