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All about engine air filters

It takes three things to make your engine run: a spark, fuel and air. Spark plugs ignite the air/fuel mixture. Of course, without gas, there's no go. And air is needed to burn the fuel. But only the air filter stands between your engine and the dirty air outside, which is trying to get in and steal horsepower.

Mechanic replacing a car engine air filter

The importance of air filters

Modern air filters block up to 98 percent of dust, pollen and other contaminants to keep your engine healthy. A high-quality air filter can grab dust particles down to 5.5 microns in diameter – compared to about 50 microns for the average human hair.

If the car’s air filter isn't doing its job, then dirt can enter the engine and make its way into the oil supply over time. This can wreak havoc on expensive internal components of the engine, such as the valves and valve seats, piston rings and cylinder walls.

Even before causing expensive damage, a dirty air filter can reduce the airflow into the engine, robbing your vehicle of power, reducing performance and fuel economy. That's why some racecars run without any air filter; they'd rather risk damage to the engine than risk reducing horsepower and throttle response from constricted airflow through a dirty filter.

Engine air filter design and maintenance

Air filters are typically made of a paper-like fiber with accordion pleats to increase the surface area. However, there are also foam filters on the market, and lifetime filters sometimes use oil-soaked cotton fibers. In many older domestic cars, the air filter was shaped like a ring and mounted on top of the engine. Inspecting and changing the filter was easy because the air filter cover was held on with one wing nut. Today, the air filter in most cars is usually a rectangular cartridge sealed in a plastic box that's part of a sophisticated air intake system. Still, replacing an air filter can be relatively easy, requiring you to only remove a few screws or clips.

Filter manufacturers recommend replacing an air filter every 12,000 miles, but you should also check your vehicle's owner's manual for the manufacturer's guidelines. Keep in mind that more demanding driving conditions – such as idling in heavy traffic, driving on dusty roads or towing heavy loads – can put a strain on engines, so you may need to change the filter sooner than recommended. Stop-and-go traffic around town is considered heavy-duty use, so the family soccer van may deserve the same maintenance regimen as an airport taxi.

Lifetime air filters: A potential upgrade

Manufacturers of lifetime air filters say their filters grab more particles while maintaining heavier airflow compared to paper or foam filters. Keep in mind that while lifetime filters may cost less than buying a new filter every year or so over the life of the vehicle, they demand additional maintenance. Instead of disposing of a used filter, a lifetime filter sometimes needs to be cleaned and coated in a specialty oil, which often involves more effort than simply installing a new replaceable one.

A lifetime air filter can also be expensive – $50 or more – depending on your vehicle, but if you install one when you first buy your car and keep the car for several years, you could save money in the long run. However, another downside to using these types of filters is that quick lube shops or dealers may not be prepared to service them.

Cabin air filters

Since 2001, most cars have been equipped with a cabin air filter designed to keep the air inside the car clean and free of dust, smoke and odors that may waft through even when the windows are closed. Check your vehicle’s owner's manual for the recommended replacement schedule and to find out where the cabin filter is located. Depending on the make and model of your vehicle, there may be several types of filters available, such as a particulate cabin filter or an activated charcoal cabin filter.

A particulate cabin filter works much like the filter in a home heating, ventilation and air conditioning (HVAC) system, and may be electrostatically charged to capture and hold fine particles of environmental contaminants, such as dust, soot, pollen, fungus and bacteria. An activated charcoal cabin filter has the capability to keep odors out of the car’s cabin by capturing and holding toxic and smelly gases, such as ozone, sulfur dioxide, nitrogen oxide and hydrocarbons. Often, a cabin filter may be hidden out of sight but is easily accessible under the dash, behind the glove box or in the air box in the engine compartment. Depending on the location, a cabin air filter can be changed in 10 minutes or so.

To keep your car running better – and to give you more comfort while riding in it – keep your air filters up to date.


Gary Wollenhaupt is an experienced automotive writer, writing for Ford, GM, Saab and others. He’s also a shade tree mechanic tasked with keeping the family fleet on the road.
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