What happens when you have a bad air filter?

By June 30, 2020October 18th, 2020Common Questions, Maintenance

My friend was asking me questions about bad air filters the other day. I was feeling lazy and wanted to just google up an article about it and give it to him. To my surprise, every other article was only talking about the symptoms of a possible bad air filter. What my friend wanted to know was what happens internally when you do have a bad air filter. Not just the symptoms! 

For a quick answer, a bad air filter can cause your car to have reduced performance, inefficient fuel consumption, contaminated engine oil and quickly degrade your engine parts from wear and tare. 

Interested to know the internal details? Let’s jump right in. I will keep it simple, I promise! 🙂 

What is a car air filter?

Before we can dig into the consequences of bad air filters, we must first understand – what even is an air filter? Your car engine needs a combination of air and fuel to combust and generate power. Thus, your car engine sucks outside air into the combustion chamber when it’s running. An air filter is responsible to filter out any debris, bugs or dust (impurities) from entering your engine, while allowing air to enter. Simple right?

Image of stock air filter

How is an air filter bad?

An air filter needs to balance the amount of air it allows to enter vs the amount of impurities it filters. A very restrictive filter will block out all the impurities but also not allow much air to enter. Similarly, a loose filter will allow a lot of air but not do a good job in blocking the impurities from entering your engine. 

The most common type of bad air filter is a dirty filter. A stock air filter gets dirty after about 15,000 miles, where they should be replaced. A dirty air filter is usually full of debris, dust and bugs stuck on them. This will block out the majority of incoming air while also allowing the already stuck particles to enter. Totally opposite of what air filters should do! 

Image of dirty / bad air filter vs clean air filter

The other type of bad air filter is simply one that allows too little air to enter or too much impurities. Unfortunately, air filters that allow too much impurities in are quite common – the most common ones being performance air filters. 

In the automotive industry, ‘more air equals more performance’ is a common knowledge. The more air your engine gets, the lesser fuel it needs to combust and produce the same power. Thus, more air + same fuel = more power. Air filter manufacturers  are well aware of this. Thus, they produce subpar air filters that allow more airflow but also let too much impurities in. They then brand it as ‘performance air filters’ and increase the price by 400%. 

Don’t get me wrong though. Not all manufactures do this. Some performance air filters do give an added horsepower. I covered performance air filters in this article. Check it out. 

Image of Performance Air Filter

Impacts of bad air filter

So we have learned what a bad air filter is. Now let’s take a look in detail, at what actually happens when you run with a bad air filter.

Reduced Performance

Remember I said more air + same fuel = more power? Well, the reverse also works. Less air into the combustion chamber, less power is generated. Your car engine produces power by creating combustion using a mixture of air and fuel. Less air with the same amount of fuel will reduce the power generated. 


Inefficient Fuel Consumption

This point is similar to the one above. I mentioned that less air with the same amount of fuel will reduce the power generated. But what if you still want the same power? If you continuously press hard on the paddle with a bad air filter, your engine will have no choice but to inject more fuel to deliver you that power. Which inevitably leads to an inefficient fuel consumption. It’s all about the balance when it comes to air filters and performance! 


Wear and Tear on Engine Parts 

So far we have talked about the problems with less air. What about the problem where impurities enter? Where do they go? The impurities that go past the air filter could end up deep into your engine. Even into the combustion chamber. Along the way, these impurities could get stuck onto engine parts like valves, pistons and cylinder walls causing wear and tear damages along the run.


Engine Failure

The wear and tear damages on engine parts are the most common and they are the best scenarios. In the unlikely case, the impurities that go past the air filter could cause extreme damage or even total engine failure. This is more common on cars that use turbochargers to deliver performance. You see – turbochargers deliver pressurized air into the combustion chamber to produce more combustion. If impurities such as metal enter the turbocharger, it will likely cause extreme damages. 

Symptoms of a bad air filter

So we have spoken of the impacts of bad air filters and even go detailed. But how could you spot a possible bad air filter and fix it before more damages are done? Let’s talk about the symptoms now. 


Dirty air filter

Well… this is a no brainer. If your air filter appears dirty, then it probably is a bad air filter. Dirty air filters do not allow much air flow in, while also allowing impurities to enter. Replace that air filter bro! Remember, replace after every 15,000 miles for a stock air filter. 


Inconsistent Idle

If your car is on idle and you feel weird vibrations, then your air filter might be clogged. This is also known as a car hiccup or cough. A car hiccup is caused by a bad spark plug, unable to ignite the air and fuel mixture inside the combustion chamber. In most cases a bad spark plug is caused by a bad air filter – causing the spark plug to be clogged and coated with dirt. 


Black smoke

The lack of air in the combustion chamber can cause some of the fuel to not burn off completely. This is apparent when you see black smoke exiting your car exhaust. When this happens, it’s also common to see flame at the end of the exhaust pipe. This is due to the surrounding flame re-igniting the unburnt fuel. This might look dangerous because it is! Bring your car to your mechanic as soon as possible when you see this. 

Image of exchaust with black smoke

Fuel Smell

This symptom is related to the above point. The unburnt fuel from the combustion chamber will exit through your car exhaust. Causing you to smell the fuel. 

How to fix a bad air filter?

Fixing a bad air filter is usually easy. If you are using a stock paper based air filter, then simply replace it with a new one. Here’s a paper based filter from Amazon for $22. If you are using a performance air filter that’s coated with oil, then you will need to wash it and re-oil. This process is also quite simple. Here’s a YoutTube video demonstrating how to clean a performance air filter.

If you do not wish to fix your air filter yourself, don’t worry. This is a quite common process. Any mechanic shop should be able to help you with ease and low cost. 

How do I maintain my air filter?

So far we have seen the potential damages from a bad air filter. To avoid these risks, ensure you regularly maintain your air filter. It’s not hard! 

Here are some quick pointers to ensure your air filter is in good condition: 

  • Replace your stock air filter after every 15,000 miles
  • Clean your reusable air filter after every 30,000 miles
  • If possible, avoid driving in dusty areas like off road 
  • Regularly wash your car 


In this article, I explained how your car sucks air from the surrounding to create combustion and deliver power. Air filter is required to keep out any impurities such as dust, debris and bugs from entering your engine. 

An air filter is considered bad if it allows too little air or excessive impurities to enter. A dirty air filter is the number 1 culprit of bad air filters. Other than that, alot of performance air filters attempt to allow much more air flow, in exchange of allowing too much impurities to enter. This too, is an example of a bad air filter.

A bad air filter can cause various problems. Some are common while others are dangerous. 

  • Reduced performance
  • Inefficient fuel consumption
  • Wear & tear on engine parts
  • Engine failure

Most of the time, drivers do not know they have a bad air filter – causing them to still equip the bad air filters for a longer time. Here are common symptoms that could indicate a bad air filter: 

  • Dirty looking air filter
  • Inconsistent engine idling AKA Car hiccups
  • Black smoke from exhaust pipe
  • Fuel smell

Thankfully, fixing a bad air filter is as simple as it gets. For a stock paper based air filter, simply replace them. For a reusable air filter, you need to take them off, wash them and re-oil if required. 

I hope this article helps you in understanding the impacts of a bad air filter. Just knowing the symptoms may not be enough. You must know what’s happening inside your engine 🙂

Make sure to come back often. We post useful blogs like these on the daily. See ya!

Ifandi L.

Ifandi L.

Passionate about everything mechanical. Ifandi has been involved with motorcycles and cars since the old days - in his family's auto parts shop. Want to keep in touch? Scream "STRAIGHT PIPEEEEE" at the top of your lungs and Ifandi will show up.

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