What Are The Signs Of A Bad Radiator Cap?

Even after being so tiny and simple, a radiator cap plays a vital role in keeping the cooling system of your car to the recommended pressure level. But, things will not work in the same way if the cap is a faulty one.

So, if you have a car, whether large or small, ordinary or luxury, you should know the signs of a bad radiator cap to replace the cap at the right time.

Otherwise, the coolant would leak randomly, causing greater damage to the hoses, reservoir, and engine.

Major Signs Shown By A Worn Out Radiator Cap

Go through each of the following and make sure that your cooling system doesn’t have any of the symptoms. If it shows at least one of them, be agile enough to inspect the system. And start with the radiator cap.

Leaking of coolant

It is the radiator cap that makes the coolant stay inside the engine where it should be. Usually, coolant will never leak unless and until something is wrong with its protective part – the cap.

This generally happens to old vehicle models. Other than the radiator cap, the coolant can also come out of the water pump.

Overheating of the engine

The overheating of your engine can be caused in two broad ways:

First, the radiator cap might be failing to maintain the pressure inside the cooling system.

Secondly, air pockets inside the cooling system might not allow the coolant to function naturally.

Now, the question is – how is the radiator cap responsible for the second way?

Air pockets are created only when air can flow inside the cooling system and make room for it. And, the air can flow inside only if the cap isn’t tight enough.

Visible steam

Coolant is boiled and evaporated, which looks like white vapor. This evaporation occurs in such a small volume that the steam is generally invisible or slightly visible.

But, if you start seeing steam in larger volumes, this will mean that the cap of the radiator is not holding the boiled coolant inside.

Collapsed/leaked hose of the radiator

Hoses attached to the expansion tank and radiator conduct the coolant via the cooling system so that your car engine can have an optimum temperature.

Unfortunately, this significant addition collapses when the radiator cap is faulty.

How can you say whether the hoses are collapsed or not?

Rember that a collapsed hose seems like a flat tire.

If this condition of the hoses goes unchecked by you, the pressure inside the cooling system will keep going higher. You will soon start to see holes and cracks in the hose.

Drop in the coolant level

There is a recommended coolant level for your car, isn’t there?

If you see the level of coolant in your car dropping, this is a big sign that there is something wrong with the radiator cap. Yes, it can also be caused by other malfunctions. But radiator caps are one of the biggest causes of this problem.

White line

Is there a sudden appearance of white line or streak on the radiator cap? It is another symptom of a cap failure. The line is created by the constant leaking of coolant that you mightn’t have noticed.

Overflowing of the reservoir

As coolant expands a lot, it often flows into the reservoir tank. This builds extra pressure upon the reservoir. To reduce the pressure, radiator diverts some amount of coolant to the overflow tank.

But, if the cap itself is faulty, it fails to perform as expected. As a result, coolant gets released quickly and goes inside the reservoir tank willy-nilly. This eventually causes the reservoir to overflow.

Things To Be Considered Seriously

While buying the cap or using one, you should check and maintain certain things. Doing this you can have the right cap or prolong the service ability of the existing.

Here follows a series of recommendations from our part:

  • Always be aware of the pressure for what your cooling system is designed for. Have a radiator cap that can withstand the exact level of pressure.
  • Stop driving the car or the engine as soon as you feel like the engine is getting overheated. Otherwise, you will end up causing major engine failure.
  • Check the hoses frequently to find if they are too hard or soft. Hard hoses have a greater chance to burst, while the soft ones may collapse at any time.
  • Avoid using water instead of coolant as much as possible because water containing minerals can corrode the cooling system.
  • It is never wise to remove the radiator cap and open the engine hood before the system gets cold naturally.
  • If you’re somehow not able to replace the cap soon, at least clean it and its adjacent area using water and a tiny brush.
  • In case you don’t find any kind of problem with the radiator cap, don’t forget to check the other relevant parts, including hoses and reservoirs.

Doing or maintaining the recommended things, you can boost the performance and overall lifetime of the radiator cap, cooling system and engine.


If you still have some unique questions lurking in your head, you can look for the answers here.

1. How often should a radiator cap be replaced?

Ans. There is no limited lifetime of the cap. But, you should replace it instantly as it starts performing unnaturally.

2. Can a bad radiator cap cause bubble?

Ans. Yes, a malfunctioning cap often causes bubbles inside the expansion reservoir by allowing air to enter into it.

3. How much does it cost to replace a bad radiator cap?

Ans. The replacement can cost you between $10 and $250. The exact pricing depends on the type of the cap and its pressure holding capacity. Depending on the model of the vehicle.

4. Does a new radiator come with a cap?

Ans. Not all radiator offers you the cap.

Final Words

To conclude, by learning the signs of a bad radiator cap, you can deal with your car more professionally. Keeping the cooling system and coolant under control, you help the engine perform better for an additional period of time.

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