Signs of A Bad Radiator Cap

Signs of A Bad Radiator Cap – The radiator cap plays an important role in your car just like the engine, brakes and transmission does.  They ensure a smooth drive and safety for your car.

A radiator cap is more important than the way people see it. It might look quite simple or insignificant, like just an ordinary lid for the reservoir, but in the real sense, it is critical for properly operating the cooling system and is often overlooked as the cause of the engine overheating.

The radiator cap is not just an instrument for opening and closing a reservoir or radiator, its usefulness is far beyond that. It serves as a pressure release valve and as a vacuum seal to your car’s cooling system.

What is A Radiator Cap?

A radiator cap or radiator pressure cap is a small mechanical component that controls the pressure of the coolant in the radiator.

The radiator cap does not only keep liquid in, despite its small size, it is designed to improve the working condition of the engine’s cooling system.

The radiator cap controls the expansion of the engine’s coolant and provides constant pressure on the system. It allows the coolants to contract and expand without letting air into the cooling system.

The radiator cap pressure rating varies by engine application and it is between 13-16 psi. High-performance radiator caps range from 19-32 psi.

How does A Radiator Cap Work?

A radiator cap consists of thin sheet metal at the top and other functional parts like the valve, spring, flat rubbers and a thin rod.

The radiator cap increases the boiling point of the coolant in the reservoir just in the same way a pressure cooker increases water temperature when boiling.

As the coolant heats up and expands, it causes pressure to build up in the radiator. This pressure then suppresses the boiling of the coolant.

The radiator is covered by the cap’s seal and a spring bears the cap and holds down the pressure valve which the vacuum spring and valve opens forcefully when the pressure is up to 15 psi allowing hot coolant to escape to an overflow tank in the cooling system. The cap is the only available place for this pressure to escape.

When the engine cools, a vacuum is formed on the top of the radiator which allows coolants that were saved in the overflow tank to be drawn back from the overflow into the radiator.

Amidst all of these processes, it is the radiator cap that regulates the maximum pressure which prevents damage to the hose and the radiator itself. The radiator cap has return valves that absorb coolant from the overflow tank to ensure the engine cools down and the coolant contracts significantly.

Signs of A Bad Radiator Cap
A Radiator Cap

What are the functions of a Radiator Cap?

The radiator cap is very functional and plays several roles in your vehicle.

Some functions of the radiator cap are listed here.

  1. When the coolant expands as it gets hotter, the radiator cap allows the coolant to escape to the reservoir tank to ensure the radiator contains only the adequate amount of coolant when it expands and the coolant is returned when the radiator cools down.
  2. The radiator cap ensures the radiator pressure remains at 16 psi to prevent the coolant from burning off when it gets very hot. The radiator cap keeps the cooling system pressure by adding pounds of pressure to the system to increase the boiling point to 260 degrees.
  3. Air in the cooling system will cause corrosion to the system components and improper cooling. Hence, the radiator pressure cap plays a role in preventing the absorption of air in the cooling system.
  4. The radiator cap keeps contaminants away from the cooling system by keeping it sealed.
  5. A radiator cap acts as a pressure relief valve that prevents excessive pressure which could damage the heater core, hoses and water pump seal and prevent them from collapsing.

Signs of a Bad Radiator Cap

A radiator cap is an important component of the cooling system, which is most times overlooked as a possible cause of engine overheating.

A bad radiator cap can lead to loss of coolant from the radiator, engine heating up, or major damage to the engine.

The main signs of a bad radiator cap include:

Leaking coolant

The radiator cap ensures the coolant stays where it is supposed to be. If the coolant is leaking out, it means the cap is bad.

If an attempt is made to use a radiator cap that holds more pressure than it was originally designed for, it may cause a leak.

In a normal working condition, the radiator cap does not permit excess pressure to get into the coolant reservoir when it reaches the required pressure. However, if the cap is bad, too much pressure  might get in and  build up in the radiator. When excess pressure builds up, it can cause seals, hoses or any joining in the radiator to break, introducing a leak in the cooling system.

When pressure builds up in the radiator, it can cause seals, hoses or any joining in the radiator to break, introducing a leak in the cooling system.

Overflowing reservoir coolant

There is a reservoir tank on one side of the cooling system and as explained earlier, the coolant in the radiator expands as it gets hot, increasing the volume of coolant in the radiator.

This increased volume of coolant cannot be contained in the radiator, thus, the excesses are moved into the overflow tank. However, if the radiator cap is bad, the coolant is released before time and it flows prematurely into the overflow tank.

Engine Overheating

When the coolant leaks out of the cooling system, there is insufficient pressure or there is air inside of it, the engine will get very hot.

Insufficient pressure in the radiator lowers the boiling point of the coolant making it unable to absorb heat from the engine. The temperature in areas close to the exhaust and spark plug usually goes above 700 degrees while the engine is running.

This overheating can cause damage to the cooling system and other engine components.

The collapse of the radiator hose

If the radiator cap is not working properly, the radiator finds an opening where pressure can escape because the cap at this point cannot release pressure to create a vacuum.

Radiator hoses now become the only available point and they are weak and easy to invade because they are attached to the radiator with just a clamp. As pressure builds up, the clamp opens causing the radiator hose to collapse or burst.

Steam from the engine

As coolant boils due to high pressure and increased engine temperature, it is released in a gaseous form as steam. If your radiator cap is faulty, maybe as a result of a bad seal, the coolant will boil and escape as steam through the narrow openings or when you open the radiator cap.

White stripes over the radiator

When the coolant is leaking, it doesn’t go directly to the ground, rather it comes in contact with other components of the engine. But the engine’s surface is hot when it is running and when this coolant touches the hot surface, the chemical composition of the coolant causes it to react on the hot metal surface leading to the appearance of white stripes over the radiator.

Auto engine cut off

Excessive high temperature in the engine will cause it to malfunction.

A faulty radiator cap will cause excess pressure to be built inside the radiator, and the heat sensor and temperature gauge will cause the engine to shut down automatically.

Air inside the engine

If the radiator cap is bad, air can escape into the cooling system through any available opening, like a broken seal or hose and creates air pockets in a vacuum.

Whereas, a functioning radiator cap will create a vacuum and achieve pressure in the right volume. When the air has taken over the place of coolant in the engine, the result will be an overheated engine.

What problem can a bad Radiator Cap Cause?

If your radiator cap is bad, it can cause several problems to your cooling system or engine as a whole and will in turn affect the overall performance of your vehicle.

Some of the problems include:

Collapsing of the radiator pressure cap

The internal components of the radiator cap are made from brass and rubber.

This brass can fracture or the rubber seals can wear out causing leakage and overheating when the engine is running.

Inappropriate pressure

The normal pressure for low-pressure caps ranges from 13-16 psi.

If the pressure on the cap is too much, it will strain radiator seams, and blow up hoses and core plugs. This is why it is important to use the appropriate cap for your radiator which will give it the appropriate pressure.

Springs rust

If the radiator cap is bad, the springs will rust, weaken and break.

This is because the system no longer puts up the normal pressure and coolant will be lost which will cause the engine to overheat.

Overheating engine

The engine plays a huge role in the performance of your car.

If the radiator cap is bad, it will cause the engine to overheat due to insufficient pressure or the presence of air in the cooling system.

How to Test a Radiator Cap

You can test the radiator cap with these steps.

1. Allow the system to cool then open the hood and remove the cap

It is important to remove the cap only when the engine is not running and must have cooled down.

A damaged seal is one common problem of the radiator cap which will introduce leaks to the system. As such, it is important to inspect the seals for any damage.

Another thing to look out for at this stage is a broken recovery valve and rusted springs. The recovery valve is a small round piece at the center of the pressure cap.

The seal and recovery valve need to be inspected because if either of it is faulty, there will be a problem with the radiator cap.

1. Install the radiator pressure cap into the radiator cap adapter supplied with the tester set

The adapter resembles a radiator filler neck on both ends. There are different sizes and shapes of radiator caps as well as adapters.

All you need to do is to choose the adapter that has the same shape as the radiator filler of your car.

Install the radiator cap to one end of the adapter and the other end of the adapter to the pressure tester to check the level of pressure in the cap.

3. Screw the adapter till it tightens

You must endeavor to screw the adapter to become very tight to ensure there is no space for pressure to leave the system.

This will also create the appropriate vacuum in the system.

4. Once it has reached the normal pressure reading marked on the radiator cap, stop adding pressure and wait for 15-20 minutes to see if the system holds pressure.

If there is a pressure drop, then there is a leakage somewhere, if otherwise, then it could be that the pressure cap has gone bad without leaking or bursting.

At this point, you will need to change the radiator cap.

How to replace a Bad Radiator Cap

Replacing a bad radiator is very simple and might only take a few seconds of your time.

Below are the steps to replace a radiator cap:

  • Leave the engine to cool down for some minutes—about 30 minutes after shutting it down before touching the cap to avoid burns or spilling of hot coolant on your skin.
  • Even after leaving it to cool, use thermal-resistant gloves and a cloth to unscrew the cap because a bad radiator cap can overheat. Keep applying pressure on the radiator cap even while removing it.
  • At this stage, the replacement cap should be handy. Install the new cap on the radiator immediately after removing the old radiator cap.

Cost of replacing the Radiator Cap

The radiator cap is one of the cheapest car components to replace if it is bad.

The radiator cap costs about $15-$50 and you can easily get it from an auto parts store, then replace it on your own following the procedure stated above.

Frequently Asked Questions

When should a Radiator Cap be replaced?

You might not need to replace your radiator cap frequently because it does not go bad easily. You can replace the unit after 5 years to avoid issues that might pop up along the line. Also, if you notice any sign of a bad radiator cap as discussed earlier, you should replace it ASAP to prevent further issues.

What happens if the Radiator Cap is loose?

The radiator cap helps to retain enough pressure in the system. If it is loose, the system will not have enough pressure which will lead to overheating. Without pressure, the coolant will boil away and cause damage to the engine. It can cause overheating, engine cracking or warped cylinder heads.

How can I tell if my Radiator Cap is faulty?

You can tell that your radiator cap is faulty if you’ve noticed any or some of the signs listed earlier. Your engine will overheat, coolant will leak, the radiator hose will burst or collapse, white streaks will appear on the engine surface or steam may emanate from the engine. You will experience any or some of these symptoms or if the radiator cap is bad and it is important to have it replaced as soon as possible.

How long can I drive with a faulty radiator cap?

The radiator cap is a very sensitive component of the cooling system and can cause problems if it is faulty. It is important to replace it as soon as you start encountering problems from a bad radiator cap. You can only drive your vehicle for about 50-60 miles if the radiator cap is bad and this too should be with a very limited speed to avoid unexpected vehicle failure. The fact is that it is unsafe to drive with a bad radiator cap.

How often should I change the Radiator cap?

As recommended, a radiator cap can be replaced every 40,000 miles although there is no specific lifespan for it. Some essential components of the cap will begin to deteriorate over time, the springs for instance will loosen while the rubber seals will wear out. Other components like the metal valve might wear out too. You need to inspect and change the radiator cap once it starts developing faults.

Can a bad Radiator Cap cause bubbles?

Yes! If the radiator cap is bad, bubbles will build up inside your radiator. This is because air will get into the cooling system through openings caused by the damage and the normal pressure of the radiator will be disrupted. Bubbles in the radiator will lead to an overheated engine.

What does a bad Radiator Cap look like?

A bad radiator cap will be quite different from a good one. You might notice some puncture or cuts on the rubber seals. If coolant is leaking from the cooling system, there will be white stripes on the radiator cap, the inner and outer surface of the radiator may get very rusty. Also, if the spring-loaded mechanism is faulty or rusty, the radiator cap will give a piercing sound when you press it down.

Is the Radiator Cap supposed to be hot?

Yes! The radiator cap will always be hot when you turn on the engine and even some minutes after the engine has been turned off due to the combustion process that takes place in the engine. The coolant in the cooling system helps in reducing heat during combustion. The heat that has now been absorbed by the coolant heats other components of the cooling system like the radiator cap and hoses.

What happens if I start my Car with the Radiator Cap off?

It’s not advisable to start your car with the radiator cap off. The first thing that will happen if your car is running with the radiator cap off is that air will get into the cooling system and cause overheating. Secondly, you will lose almost all the coolant from the cooling system because it will boil out as a result of an overheated engine. Starting the car without the radiator cap may cause damage to your engine. It can blow up the head gaskets. The engine will overheat and go bad because it cannot maintain a consistent temperature at this point.

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