9 Symptoms of Bad Radiator Cap + How to Fix

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symptoms of bad radiator cap

Many people do not give due importance to the radiator cap. A radiator cap is often neglected and considered an ordinary reservoir covering.

But it is hardly a standard instrument of opening and closing a reservoir. It acts as a pressure release valve and as a vacuum seal by working as a spring-loaded mechanism.

Recently a friend shared that he was facing the issue of leaking coolant in his car. He was about to inspect his car for any internal or external leaks.

I advised him to check for a bad radiator cap first as it can also cause a coolant leak. I gave him insight into other bad radiator signs. The next day, he returned with a positive response.

So, I thought maybe it would be good to share all the information I’ve learned on radiator caps.

What is a Radiator Cap?

cooling systems

It is a metal cap placed on the top of a radiator. Plus, it is a mechanical device mainly responsible for maintaining the correct amount of coolant. It is also responsible for keeping the cooling system’s pressure in check. The normal pressure range of the cooling system is 15 to 20 Psi.

What is the Function of a Radiator Cap?

In addition to these basic functions, the radiator cap also performs many detailed roles.

1. Letting coolant into the Overflow Tank

When the engine of the car is running, it becomes hot. The function of the coolant is to circle the engine and absorb this heat. The volume of the hot coolant expands in comparison to the cold coolant. Tubes of the cooling system cannot hold this increased volume of coolant.

This is where the radiator cap comes into play. It allows the excess coolant to move into an overflow reservoir. Hence, ensuring the smooth function of the cooling system.

2. Radiator Cap as a Pressure Relief Valve

The radiator cap serves the same purpose in the cooling system as the pressure knob serves in the pressure cooker. High temperatures build too much pressure inside the cooling system. Excessive pressure can cause the failure of sensitive cooling system components.

This is when the radiator cap comes to aid. As radiator cap is the only component in the complete cooling system that can release the pressure. So, the spring-loaded valve of the radiator cap is lifted, and the pressure drops.

3. Allow Coolant to Contract and Go Back

When the engine is turned off, the average temperature drops down instantly. The job of coolant is completed and there is no more need to carry away the heat.

Again the radiator cap acts as a sensing element. It notices that the pressure in the cooling system has dropped. As the coolant contracts, the opening of the radiator cap allows it to go back into the coolant overflow tank.

Construction of a Radiator Cap

On the outside radiator cap looks like any other typical cap. But on the inside, it has a spring-loaded mechanism. The basic components of this mechanism are spring, valve, thin rod, and flat rubbers.

This mechanism allows the radiator to open up when the pressure is building up in the cooling system. Similarly, it also enables the creation of a vacuum inside the cooling system by cutting off the air supply to it.

Signs of a Faulty Radiator Cap

heater core

Following are the symptoms of a bad radiator cap:

  1. Leaking Coolant
  2. Overheating Engine
  3. Radiator Hose Collapses
  4. Overflowing Reservoir Coolant
  5. White Stipes Over the Radiator
  6. Air Pockets Inside Engine
  7. Radiator Hose Bursts
  8. Steam From Engine
  9. Auto Engine Cut Off

Let us explain the symptoms of a bad radiator cap in detail:

Leaking Coolant

Among all symptoms of a bad radiator cap, leaking coolant is the clearest one. Since the basic function of a radiator cap is to maintain the pressure inside the car’s cooling system. If the radiator’s cap is faulty, then the chances are that the pressure will build up inside the system.

This can cause any seal, any joining of the system to leak. As coolant is the running fluid inside the entire system, hence it will come out of these openings.

If you have low coolant, then you need to check your radiator cap first.

Overheating Engine

When the engine is running, the local temperature at places near the spark plug and exhaust valve can go above 700 degrees Celcius. There is a dire need to keep this temperature down to stop permanent damage to core engine components.

Radiator, radiator fans, coolant, and all other parts of the cooling system work as a team to keep the engine’s temperature under check. The radiator cap is at the head of this system. If it goes bad, it can disturb the running of the entire cycle.

Overheated engine and blown head gasket are an indication that the radiator cap might have gone bad.

Radiator Hose Collapses

As mentioned above, one major function of the radiator cap is to build the necessary pressure and release pressure to create a vacuum. If it is not working properly, the vacuum will not be released properly.

It will find an opening or hole through which the vacuum can escape. Now, radiator hoses are the weakest links in the entire chain.

The easiest passage for pressure to be released is via a radiator hose as radiator hoses are attached to the main system with a single clamp.

As the pressure builds due to a faulty radiator cap, the clamp is opened and the radiator hose collapses.

Overflowing Reservoir Coolant

A reservoir tank is present on the side of the radiator. When the engine is running, the hot coolant expands. This increased volume of the coolant cannot be withheld inside the cooling system of the car.

The overflow tank is there to take the excess amount of coolant from the closed cycle to enable the smooth running of the system.

When the radiator cap is faulty, the opening will be released before reaching the peak pressure. As a result, the coolant overflows into the reservoir tank prematurely.

White Stipes Over the Radiator

The first and foremost thing a faulty pressure cap will cause is leaky coolant. When the engine coolant is leaking, it is very unlikely that it directly makes it to the ground without coming in contact with any other engine components.

The surface of an engine is scorching when it is running. When the leaked coolant comes in contact with the hot surface of the engine, the chemical composition of an engine coolant is such that it reacts with the metal surface and form white lines all over it.

Hence, if you see these white steaks over the engine, then chances are that your radiator cap might have gone bad.

Air Pockets Inside Engine

The pressure maintaining function of the radiator cap causes many symptoms to be revealed in case it goes wrong. If the radiator cap is functioning properly, then creating vacuum and achieving desired pressure is done in the right way.

However, if it goes bad, the air might enter into the cooling system via openings, and air pockets are created in the perfect vacuum.

Now, you cannot dive into the engine to see that the air has taken the place of coolant. But what you will experience is an overheated engine because air is a terrible conductor of heat compared to the coolant.

So, the cooling function of the coolant is compromised, and the engine is at the risk of things like head gasket failure.

Radiator Hose Bursts

If the radiator cap fails then radiator hoses burst very often. As pointed above, the weakest link in the chain is the radiator hose. As it has no reinforcement, just a little clamp that attached it to the main system.

Coolant hoses are made up of rubber so they can develop a pinhole very easily. Once the pressure builds up in the hoses they burst at once.

Steam From Engine

Coolant is present in the engine in the form of liquid. When the pressure builds, and the engine’s temperature rises due to a bad radiator cap, high pressure and temperature increase the boiling point of the coolant fluid.

So the coolant fluid starts boiling and is converted into a gaseous state. These vapors escape from narrow openings, or they will also come out of the radiator cap every time you open it. If you notice these white fumes, it indicates a bad radiator cap.

Auto Engine Cut Off

A bad radiator cap can cause automatic engine cut-off in your vehicle as well.

High temperature does not sit very well with car engines. When pressure is built inside them due to a faulty radiator cap, the heat sensors and temperature gauge will shut off the engine automatically, so that the adverse effects of increased temperature like failed head gasket can be avoided.

How Can You Test a Radiator Cap?

The best way to test a bad radiator cap is to use a radiator pressure tester. It can also be used to test any other components of the coolant system, plus it will also determine the leakage points in the coolant path if there are any. Let us take a detailed look at how to conduct a pressure test.

  1. The first step is to arrange a radiator pressure tester. You can either buy it or rent it for some time.
  2. If you have just used the vehicle, wait until the engine cools.
  3. Open the hood of your car.
  4. Once the engine has cooled down, now open the radiator cap very carefully by pushing it downwards and rotating it counter-clockwise.
  5. Then you need to find the right cap adapter according to the size of your radiator.
  6. Now place one end of the cap adapter on the pressure tester inlet. Screw it carefully and ensure that no fluid can pass through it.
  7. Place the other end of the adapter on the radiator neck.
  8. Screw the cap adapter until it has become tight. Give it a twist or two even after it has become tight to ensure that there is no way any pressure leaves the system.
  9. Now perfect vacuum should have been created in the system. And if this is not the case try to find some other adapter for your radiator.
  10. Turn on the pump electric supply and by using a knob or handle on the pump slowly build up the pressure.
  11. Stop increasing the pressure once it has reached the pressure reading marked on your radiator cap.
  12. Now wait for 15-20 mins and notice if the system holds pressure.
  13. If you notice a pressure drop then look for leakage.
  14. If there is no pressure drop and still you are observing other symptoms, then chances are that the radiator cap has gone bad. But it has not caused any leakage or bursts so far.
  15. You must change the radiator cap immediately in this situation.

Replacement Cost of Radiator Cap

A radiator cap is one of the cheapest parts you can replace in your car. It costs only $15 to $55 to replace your car’s radiator cap depending upon its make and model.

You can easily get your car’s radiator cap for $15 to $50 from any auto parts store. It takes less than 30 seconds to remove the old cap and replace it with a new one. So, the labor is free most of the time.

You can easily replace the radiator cap yourself. But make sure to follow the precautions that are mentioned above to open the radiator cap.

Tips for Opening a Radiator Cap

The coolant inside the radiator is boiling when the engine is operating. So, it is very dangerous to open the radiator cap without any preventive measures. You must keep the following things in mind before touching or opening the radiator cap:

  1. If you have a faulty radiator cap, do not open the hood for at least 10 minutes after shutting off the engine. There can be some leakage and boiling hot coolant can leak onto your body or hands.
  2. Even if all indicators are normal wait for 3-5 mins before opening the hood.
  3. Wait for 45 mins before opening the radiator cap.
  4. Always use thermal-resistant gloves and heavy cloth to open the radiator cap.
  5. Keep pressure applied on the radiator cap even after unscrewing it. As sometimes it can burst open with pressure.
  6. Gently release pressure and remove the radiator cap.


People do not give due importance to radiator caps sometimes. But it is a highly critical component of a car’s cooling system.

It is not just an ordinary cap; rather it is a whole mechanical device. A sophisticated piece of machinery that can act as both a pressure release valve and vacuum pump.

If it goes bad you can commonly face problems like coolant leak, overheating engine, cooling system component failure, excess pressure, and collapse of radiator hoses.

You can also carry out a detailed radiator pressure test to check for any coolant leaks or to find out which component of the coolant system has gone bad.

None the less it is a critical component that needs to be replaced immediately if it goes bad. Special care and protective gear must be used before opening the radiator cap.

Frequently Asked Questions

How long can you drive with a bad radiator cap?

You should not drive more than 50 to 60 miles with a bad radiator cap and that too at limited speed. The reason why I say this is because the radiator cap is a mechanical hardware device. And a very sensitive one as well. It can fail at any instant and cause big trouble for you. Hence, you should get a new radiator cap as soon as possible.

How often you should change the radiator cap?

There is no defined lifespan of a radiator cap. But you should replace it after every 40,000 miles regularly. As the working components of a radiator cap include a spring-loaded mechanism that is bound to become loose after some time. Hence, it is better to stay on the right side of things.

Is the radiator cap supposed to be hot?

Yes, even if you turn off the engine radiator cap will stay hot for some time. The very function of the cooling system is to carry away the excess heat produced as a result of the combustion process. This residual heat stays in the coolant and as the coolant runs past hoses and caps it heats them as well.

It would be best if you were very careful before opening or touching the radiator cap. For detailed precautions read the above section on tips to open the radiator cap.

What does a bad radiator cap look like?

Looks of a bad radiator cap will be different from a good one. Due to leakage of coolant, it can have white stains over its surface. Similarly, if you will press down a bad pressure cap it will give a piercing sound. Indicating that the spring-loaded mechanism has gone bad or simply the parts have become rusty.

Can a bad radiator cap cause air bubbles?

Yes, a bad radiator cap can cause a bubble to build inside the cooling system of your car. Disturbed pressure or air entering the coolant path via openings can cause bubble formation. It is very bad for your engine as it puts it at the risk of overheating.

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