Bad Radiator Cap Symptoms & When Need To Replace It and Cost?

Bad Radiator Cap Symptoms

​One of the most common problems with radiators is a bad radiator cap. This problem can come with several symptoms that could ultimately damage your car and leave you stranded in the middle of the road.

Luckily for you, there are ways to know when you have a bad radiator cap before anything like this happens.

Knowing how to test your car for a bad radiator cap will save you a lot of money, time, and may even prevent you from getting stranded or worse.

In this guide, we will help you learn more about this problem and how to avoid it in time before anything unwanted comes your way. If you are interested to know more about this issue, then come further into this article and find out!

What is a Radiator Cap? 

While its name says it is simply the cap or lid of the radiator, this object has a slightly more critical job. This cap holds the coolant inside the radiator, but it also works as a two-way valve that keeps the right pressure inside so the radiator does it work correctly.

This pressure is important because it increases the boiling point of the coolant inside the radiator. Normal boiling point would be 212 ºF, but when you add the pressure inside the radiator, this number rises exponentially and allows the liquid to travel freely around the radiator without vaporizing.

Most radiators have a pressure of about 15 PSI, but you may also find some radiators with pressures from 12.5 PSI to even 22 PSI depending on the model and the type of car. The purpose of the cap, then, is to maintain this pressure inside the radiator.

This will make the radiator work correctly and prevent not only the coolant to boil down but also prevents it from leaking.

So, when the radiator cap breaks down, you will undoubtedly have issues with the radiator. But before any potentially damaging problem arises, you’ll start to experience some symptoms.

5 Symptoms of a Bad Radiator Cap

Now that you know what the radiator cap does, it will be wise to learn what signs you can notice when it is broken or defective.

The following symptoms will let you know that you probably have a bad radiator cap needing replacement:

1. Air Enters the System

You won’t notice that air enters the radiation system where the coolant is until you see cracks in the tubes. This happens because the pressure eventually falls down too much when the vehicle cools down, causing the whole hose system to warp or break.

Sometimes this occurs due to a tube leakage or simple cracks letting air inside. But most of the time it happens because the cap is defective.

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2. Low Coolant Level

There are many causes why the coolant levels in the radiator may be low. But sometimes it is because of the radiator cap merely defective. This typically comes with a warning light that signals low coolant levels in the dashboard, depending on your car.

And this happens because the cap is leaking or changing the pressure and overflowing, which eventually boils down the coolant.

3. Coolant Leaks

There’s not a more telling symptom that your car has a bad radiator cap than coolant out of it. But leaks can happen from cracks or broken tubes & hoses, yet most of the time it is the cap that’s not sealing the right way.

This eventually makes the pressure inside get crazy and leak coolant.

Something else that could happen is a radiator cap that’s stuck. This could make the hoses leak too, by building pressure inside that makes the coolant get out. If you see any leak, from anywhere in the radiator, check the cap first.

4. Overflowing Reservoir

The reservoir tank in the radiator is where the coolant goes when temperature and pressure are high. This makes the coolant expand, which makes a lot of extra pressure inside the radiator.

When this happens, the cap will release pressure to prevent the tank from overflowing. But sometimes, the radiator cap releases the pressure too soon, causing the radiator to have little to no pressure.

When the radiator cap doesn’t release pressure at the right moment, the reservoir tank will overflow. So, if you experience coolant being discharged too fast or a reservoir tank overflowing, it is probably the cap that’s faulty.

5. Overheating Engine

Whenever any of the previous symptoms appear, it is not long before the whole radiator stops doing its job that’s just cooling down the engine.

Whether it is because there’s too little coolant, the reservoir tank is overflowing, there’s coolant leaking, or the hose & tubes cracked or warped – the ultimate symptom will be an engine overheating.

If this happens, you may get stranded in the middle of the road. Your car may burn and produce smoke. Sometimes it just heats up and stops by itself. Or sometimes you’ll just see how the temperature is too high, and that’s it, in which you’ll have to stop the car yourself before it is too late.

If you want to prevent overheating from happening, next, we’ll help you test a bad radiator cap.

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How to Test for a Bad Radiator Cap

If you experience any of the previous issues with your radiator, it’s time to make sure before replacing it. Here, we will help you check your radiator cap in search of any potential damage:

Testing with Warm Engine

After shutting off the engine of the car, the radiator will keep working. You can let it sit for a minute or two and then try the following:

  • 1Look for the radiator cap and check in search of any liquid around the edges. Sometimes, bad caps start bubbling coolant which you can hear.
  • 2At this moment we only want to check the cap and the radiator. Do not try opening the cap or anything, just watch in search of any leak or coolant bubbling. If you don’t hear or see anything, you can now try with the car on.

Testing with Engine Running

While the engine is on, you can see how the radiator acts. If the radiator cap is faulty, it will immediately give you some signs. Here’s how to test it:

Step One

Let the engine run for about 5 to 10 minutes then open the hood. Watch the radiator cap and check for any sign of leaks, bubbles, or anything resembling a coolant. You should see the steam of pressure coming off the cap but nothing else. If you don’t see anything, go to the next step.

Step Two

Now you need to grab a thick cloth with a glove to prevent burning your hands. Keep your arms away from any part of the engine also. With the glove and the cloth, try turning the cap without removing it.

Step Three

It is important not to take the cap off as there’s pressure inside. All the hot liquid may eventually come out and burn your arm, body or even face. So just loosen it up slowly until it feels hard without forcing it.

Final Step

At this slightly loosen state, the cap should start letting pressurized water in the form of steam or gas out. If this doesn’t happen, there’s probably a pressure leak or low coolant levels inside which means the cap is very likely defective.

Replace it as soon as possible.

Testing the Plunger

Sometimes you can check the plunger of the cap to see whether it’s doing its job. This plunger has the purpose of sucking the coolant from the overflow reservoir to the tank and so on. However, when the plunger is not working, the coolant will stay in the overflow reservoir.

To check if you just need to do a quick visual inspection in this reservoir. If you see or hear any coolant inside, it means the plunger of the caps is not working. You’ll have to replace the whole cap.

Testing Using a Pressure Tester

If you do any of the previous tests and there’s no sign of a bad radiator cap, you can do a last but more effective test using a radiator pressure tester or pressure gauge.

Here’s how:

Step One

Start by opening the hood and looking for the radiator cap. Make sure the car is cool by now, with at least 30 minutes after the vehicle has been turned off. Then you can open the cap by pushing it down then turning it counter-clockwise.

Step Two

By now you can screw the cap adapter into the pressure tester or gauge. You should screw it far down until it doesn’t turn anymore. This will leave you with the tester attached to the radiator. Then you can screw the radiator cap tightly on the other side of the tester, so you’re ready for testing.

Step Three

Now grab the pump handle of the tester and pump until the gauge fills with pressure. The reading on the gauge shouldn’t be lower than the number of PSI that the radiator cap is supposed to offer. They usually have the number on the top.

Otherwise, you’ll have to look for the right PSI of your car’s radiator in the owner manual.

Final Step

While the pressure elevates in the gauge, you have a well-functioning radiator cap. But if the pressure falls below, it means the radiator cap is not working correctly. You can take the cap out by now and re-attach it to test again.

If it’s still not maintaining the pressure, it means you have a faulty cap ready to be replaced.

When to Replace the Radiator Cap?

Knowing how to test the radiator cap in search of leaks or any problem can be easy. But there are a few other ways also to know when to place the radiator cap.

Here, we show all of them:

  • Before testing the cap in any other way, you’ll have to look closely to the primary seal, the return seal, and the pressure seal. If any of the seals are cracked, hardened or worn down, you’ll have to replace the cap.
  • Another common issue with radiator caps is when it’s not the right one for the vehicle. There are several types of radiators out there, and all of them use different PSI readings. If you have a 15 PSI cap, but the radiator works at 18 SPI – you’ll inevitably suffer a problem. Replace the cap for the right one before having an issue.
  • Check the radiator and expansion tanks before doing any radiator cap test. This will let you know if there’s a buildup or too much pressure inside which is always a sign of a bad radiator cap that needs replacing.
  • When the pressure relief or the vacuum relief valves in the radiator cap are not easy to handle (lift and spring back), you’ll have to replace the cap.
  • If the spring of the radiator cap does not offer any resistance, it’s probably worn down or damaged. This is also a sign that the radiator cap needs to be replaced.

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Cost of Replacing a Radiator Cap

If you have to replace the radiator cap, you’ll be happy to know that it doesn’t cost much. No matter what car you’re driving, the cost of one of these parts doesn’t usually go over $20 and replacing it is a total piece of cake.

However, it is always important to replace a damaged or old radiator cap with the right one. You must be sure of the ideal PSI readings for your radiator so you can prevent further issues with it.

If you get a radiator cap with less pressure than the ideal, it will decrease the system pressure and cause overheating. If the PSI is higher than the right one for the radiator, it will cause overpressure.


To end, be sure to replace the radiator cap with the right one following our advice and instructions. This way, you can prevent further problems with the radiator system and get rid of this annoying issue that could leave you stranded. Don’t let that happen with our advice.

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