9 Signs Your Car’s Air Filter Needs Changing

Your car needs to combust petrol. And as every GCSE science student will tell you, this needs three things. Fuel, an ignition source and air.

In engines, this is supplied by the petrol, diesel or other fuel source, the spark plugs, and the air is drawn through the air filter, ensuring that dirt and muck doesn’t enter into the piston chambers. Of course – all that filth ends up caught in the air filter, reducing its effectiveness.

So after 10 to 15 thousand miles, you’ll need to replace your air filter (or sooner if you drive and live in very dusty areas). What are the tell-tale signs that this job is in your immediate future?

Table of contents:

• How does an air filter work?
• What are the main signs your filter needs changing?
• Why change your air filter
• Knowing your air filter

How does an Air Filter work?

Did you know that your engine needs around 10,000 litres of air to combust a litre of petrol? That’s a lot of oxygen in anyone’s book. The air filter is designed to catch the dirt and muck, whilst still allowing good, clean air to enter the engine, which then combusts with the fuel, before being expelled through the exhaust system.

The air filter will catch everything from small pieces of muck and debris kicked off the road surface, through to small insects or large pieces of pollen. Anything that wouldn’t be good for your engine – you can rely on this part to catch.

What are the main signs your filter needs changing?

Be aware of the following symptoms. One could mean you need to inspect the filter further. See a few in combination, and you’ll be able to nail the cause a whole lot faster.

Reduced fuel economy

So we’ve told you that the engine needs air to burn the fuel. If the filter is clogged and letting less air through, the potential energy available from the fuel will be proportionally reduced – meaning it will need more juice for the same squeeze. If your car seems to be doing fewer miles to the gallon – check the filter.

Dirty Spark Plugs

If all the petrol in the piston doesn’t combust properly due to a lack of air flow from a partially blocked filter, it burns with a sooty black flame. You’ll see the results of this process all over the spark plugs, as similarly sooty marks.

Strange engine sounds

In the same way you might wheeze and gasp for breath if you have a bad cold, and your lungs are bunged up, your engine might make its own unusual noises when it’s deprived of the necessary air to function properly. Keep one ear open for popping or coughing sounds, accompanied by vibrations. As you might clear your throat with a cold, you should inspect the filter to see if you can find a blockage.

Check engine light illuminates

The check engine light can illuminate for a number of reasons. If the air filter is clogged, then deposits of soot can trigger the system. Get the diagnostic code checked out by your preferred garage or service centre, where they’ll give you trustworthy advice on changing the filter.

Dirty air filter

So you’ve experienced some of the other symptoms of a clogged air filter on our list. Take a look at the actual part. Normally new filters are white, cream or an off white greyish colour. You won’t be able to miss the dirt on the outside, if the part is badly clogged and in need of replacement. Sometimes however, there can be a significant build up of dirt in the filter paper, requiring an actual mechanic to review the part to confirm that it’s not functioning at the required level.

Reduced horsepower

Another symptom of your engine struggling for air can be found when you hit the accelerator pedal. If the car jolts forward, and doesn’t move with the usual smoothness that you recognise, then check the air filter further, to see if there’s a build up of dirt.

Black, sooty smoke or fire expelled from the exhaust

Remember how we mentioned about the spark plugs getting dirty? The same rich fuel mixture that leads to unburnt fuel in the residual gasses created by combustion, will also come out of the exhaust, if the air filter isn’t working properly. This can lead to black, sooty smoke, or particles of fuel can even ignite when they hit the hot exhaust, causing miniature explosions and fire.

Smell of fuel when starting the car

Again, that rich fuel mixture that’s a real signature problem of a blocked air filter will show up when starting the car. If your nose detects fuel odours, then this could be down to unburned fuel. Investigate the filter further to confirm your suspicions.

Misfiring engine

At its worst the lack of air to the engine, should your filter be severely blocked, can actually cause the engine to misfire, which could create problems starting the car. If you have to make a number of attempts to start due to the rich fuel mix, then check the filter.

Why change your filter?

There are several reasons why you should change your filter, including the following…

Improved fuel economy – when the filter is supplying the exact amount of air needed to power combustion in the engine, it means that every available drop of energy is squeezed out of your fuel.

Lower emissions – unburnt fuel and other particles in your exhaust fumes increase when there’s not enough air. Cut down on your emissions by ensuring your filter is operating correctly.

Longer engine life – if the engine is not running correctly, this creates increased stress on the parts, such as the pistons, the piston chambers and even the camshaft. Engine’s struggling for air are under more stress, which can reduce their lifespan. Replacing the air filter decreases this stress, and as a result lengthens your engine’s lifespan.

Low cost – air filters are made to be replaced. You won’t be breaking the bank and the money spent on your filter will soon be repaid in lower fuel bills, thanks to a better running engine.

Know your air filter

Get your mechanic to take a look at your filter when you have a service or an oil change, but remember we mentioned 12,000 miles earlier? You should not need a replacement filter every time you visit the garage even if there’s a little dirt on the part. If you suspect there’s something up with your mechanic – ask for a second opinion, otherwise you could be wasting your money.