Under hood Maintenance: Air Filters

Even during low speed operation, the engine pulls in a tremendous volume of air. This air has a great deal of abrasive particles, which must be prevented from entering the engine. The air cleaner traps the abrasive particles before they can enter the engine. In so doing, however, it clogs itself. If the air filter is not changed regularly, it can become so clogged that it limits air flow into the engine. Manufacturers specify an air filter change interval, and the change is usually a regular part of underhood maintenance.

The air cleaner has two main components. A housing provides a container for the filter element. The housing is a metal container that is typically mounted on top of the engine. Air is routed into the housing through an intake tube assembly. The intake tube assembly on the air cleaner housing shown above is a simple tube. On late-model cars, the intake tube assembly may be very complex and control the temperature of the incoming air for performance and emission control..

The filter is the part inside the housing that cleans the air. The two basic types of filters are the paper and oil-wetted polyurethane. Heavy duty filters sometimes combine both types of filter types. The filter element is made from pleated paper. The pleats provide the maximum surface area for air to pass through. A fine mesh screen is used to support the paper element and protect against the fire hazards of an engine backfire. A top and bottom seal provides an airtight seal for the filter in the housing. Sealing is important because any air that does not go through the filter on the way into the engine could contain dirt.

Polyurethane is a flexible foam-type material. It can be used to filter air entering the engine. It is usually wetted with oil to improve its filtering ability. Filters used in very dirty conditions are often made of polyurethane and paper in combination. Incoming air is routed first through the polyurethane filter, then through the paper filter.

Replacement air filters are available for most vehicles. The filter has a part number printed on the filter box. Application charts are available in auto parts stores that show what number filter fits any particular car. Application charts are often printed on the filter box, too.

Inspect, Remove, and Replace

When inspecting or changing the air filter element, first look up the procedure in the shop service manual. The manual will explain the specific procedure for removing and replacing the element.

Cars with fuel injection typically have an air filter element located in an air induction assembly like the one shown below. The filter element is located in a rectangular box called the air cleaner housing. The element may be removed by unlatching a series of clamps or unscrewing a series of screws.

Cars with carburetors or throttle body fuel injection often have a large round air cleaner assembly mounted on top of the carburetor. The filter is located inside the air cleaner housing. Remove the top of the air cleaner by taking off a single wing nut as shown below.

To inspect or change the air filter element:

First loosen and remove the latches, screws, or wing nut. Remove the cover and then the air filter element.
Carefully inspect the air filter element. You will find dirt and oil on one side of the filter element. This material has been trapped by the filter material. Any dirt and oil buildup on the filter means it should be changed.
Place the new filter element next to the old one on the work bench. Carefully compare the two filter elements. Both must have the same dimensions. The gaskets on the top and the bottom of the filter elements must be exactly the same.
Place the new air filter element in the air filter housing . Make sure the gasket surface is aligned on both the top and bottom.
Replace the cover and tighten the latches, screws, or wing nut until snug.
WARNING: The air filter gasket must fit correctly and seal properly. A leak at the gasket means that air will go directly into the engine around the gasket without going through the filter element. Abrasives can get into the engine and shorten engine life.

SERVICE TIP: A light coat of grease on the air cleaner gasket of an older car can improve the seal between the air cleaner housing and the air filter element.

Question:

How often should I replace my air filter?
Answer:

It's hard to give a specific time or mileage figure because the life of the filter depends on how much crud it ingests. A filter that lasts 20,000 or even 30,000 miles on a vehicle that's driven mostly on expressways may last only a month or two in a rural setting where the vehicle is driven frequently on gravel roads. Changing it annually or every 15,000 miles for preventative maintenance may be a good recommendation for the city driver, but not its country cousin.
Regardless of the mileage or time, a filter should be replaced before it reaches the point where it creates a significant restriction to airflow. But when exactly that point is reached is subject to opinion.

A slightly dirty filter actually cleans more efficiently than a brand new filter. That's because the debris trapped by the filter element helps screen out smaller particles that try to get through. But eventually every filter reaches the point where it causes enough of a pressure drop to restrict airflow. Fuel economy, performance and emissions begin to deteriorate and get progressively worse until the dirty filter is replaced.

Many heavy-duty trucks have a "restriction" meter on the air filter housing that signals when the filter is dirty enough to need replacing. But lacking such a device, the best you can do is guess.

Removing the filter and holding it up to a light will show you how dirty it is. If it's really caked with dirt, it obviously needs to be replaced. Trying to shake or blow the dirt out is a waste of time because too much of it will be embedded in the filter fibers.

NOTE: Many filters that appear to be dirty are in fact still good and do not really need to be replaced. So it's up to you. If you think it's dirty, replace it. If you don't think it's dirty enough to need replacing, then don't.