Filtration is all about trapping contaminants. The engine’s oil filter traps wear particles and dirt that might otherwise damage the bearings, rings and other wear surfaces inside the engine. The engine’s air filter keeps out dirt that could damage the piston rings and cylinders, or clog air passages in the throttle body idle air control circuit. The transmission filter prevents debris from jamming control valves and causing wear inside the transmission. The fuel filter stops sediment and rust particles that could plug the fuel injectors. And last but not least, the cabin air filter prevents dust, pollen and odors from entering the passenger compartment.

By stopping unwanted contaminants before they can cause trouble, filters prolong the life of the engine, transmission and fuel system. The more contaminants a filter traps, the more efficient it becomes — up to a point. If not replaced for preventive maintenance, the filter will eventually clog and create a restriction. This, in turn, may cause additional problems.

WHEN FILTERS BECOME CLOGGED
A clogged oil filter may restrict oil flow to the bearings, camshaft and upper valvetrain components. This may force open a "bypass" valve that allows unfiltered oil to flow around the clogged filter. If the oil is dirty, the contaminants will be carried directly to the parts that the oil filter is supposed to protect. This will increase oil wear and may even result in premature engine failure.

A clogged air filter will restrict air flow into the engine. The "choking effect" can upset the air/fuel mixture causing the engine to run rich, pollute and use more fuel. A dirty air filter can increase carbon monoxide (CO) emissions and reduce high speed power.

A clogged transmission filter will reduce oil flow inside the transmission. This may cause engagement and shifting problems, or cause the transmission to slip. Slipping accelerates wear in the clutch packs and can lead to premature transmission failure.

A clogged fuel filter will restrict fuel flow to the engine and may cause a loss of fuel pressure. This can make the engine run lean, misfire and lack power when accelerating. If the filter becomes totally plugged, it may stop the flow of fuel altogether causing the engine to stall or prevent it from starting. Or, it may cause a bypass valve inside the filter to open allowing unfiltered fuel to flow to the injectors. The injectors have tiny screens in its fuel inlets, which may become clogged if the fuel is dirty.

A clogged cabin air filter will restrict airflow into the passenger compartment. This may cause a loss of cooling when the air conditioner is running, or reduce heater output during cold weather. Reduced air flow may also hamper the windshield defrosters from clearing the glass during cold weather or wet weather.

FILTER TECHNOLOGY
The media inside a filter may be resin-impregnated cellulose fibers, synthetic fibers (glass and polyester), or more commonly, a composite mixture of natural and synthetic fibers. There’s a lot of engineering that goes into finding the right balance between filtering efficiency, dirt-holding capacity and flow.

Premium oil filters typically have a higher efficiency rating than standard oil filters, and typically capture more of the smaller particles (three to 40 microns in size) that cause engine wear. Long-life filters typically have more square inches of surface area to trap and hold contaminants.

Some oil filters also serve a dual purpose and help replenish oil additives to maintain oil quality. The filter contains a gel or wafer that slowly dissolves to release chemicals that help maintain oil viscosity and neutralize acids that can form sludge.

In some cases, a filter may be designed to maximize flow, as in the case of an aftermarket performance air filter. A less restrictive filter media is used so the engine can breathe more efficiently at higher rpm. Some cotton air filters have a more open design and rely on a light surface "oiling" to attract and hold dirt. These filters can often be cleaned, re-oiled and reused. But standard air filters cannot be cleaned or reused when they become clogged. Attempting to clean a cellulose fiber filter will usually damage it – and the last thing you want in an air filter is a hole!

FILTER MAINTENANCE
Vehicle manufacturers have traditionally published recommended replacement intervals for filters based on mileage. The service recommendations can usually be found in the vehicle owner’s manual, or a separate service guide supplement. But few motorists read their owner’s manual, and in older used vehicles the owner’s manual has long since disappeared from the glovebox. Consequently, many motorists have no idea how often they should replace their filters. What’s more concerning is that many motorists are totally unaware their vehicle has a transmission filter or may be equipped with a cabin air filter.

As a rule, oil filters should be replaced at every oil change. For most urban drivers, that means changing the oil and filter every 3,000 to 5,000 miles. Under ideal conditions, oil change intervals can be pushed further apart. But doing so greatly increases the risk of engine-damaging sludge forming -especially in vehicles that are used primarily for trips of less than seven miles, or ones that are driven during cold weather. Extended oil change intervals are not recommended for older high-mileage vehicles because wear increases blowby, and that increases the formation of sludge in the crankcase.

The life of the air filter depends on environmental factors as well as mileage. Somebody who lives in a rural area and drives on dusty gravel roads might need a new filter every three to six months while somebody who only drives on paved roads might not need a new filter for several years or 30,000 miles. Air filters should be inspected when the oil is changed, and replaced if the filter element is clogged and passes little light (holding a shop light behind the filter element is a good way to judge the level of contamination). Replacing the air filter once a year may not be absolutely necessary, but it does assure maximum air flow, performance and fuel economy.

The fuel filter is typically a neglected filter. It is seldom replaced until it plugs up and causes a driveability problem or the vehicle stalls. In the past, the standard recommendation has been to replace the fuel filter every two to three years or 24,000 to 30,000 miles. But many OEMs now recommend longer service intervals of 50,000 to 60,000 miles, and some make no recommendations whatsoever for replacing the fuel filter.

No filter is a lifetime filter, and eventually even the longest-lived fuel filters will succumb to sediment and rust. All it takes is one tank of dirty gas to plug up a filter. Some fuel filters are now located inside the fuel tank and are very difficult (and expensive) to replace. These filters are long-life filters, but should be replaced if the fuel pump has failed and is being replaced. In fact, new filters should always be installed any time a fuel pump, or the original fuel tank, is replaced.

Caution: The fuel system on fuel-injected engines contains residual pressure as high as 85 psi in some applications. The residual pressure must be relieved before loosening the filter. This can be done by cranking the engine with the fuel pump disabled (remove the fuel pump fuse).

Transmission filters are also forgotten filters. The filter is usually located inside the pan on the bottom of the transmission. On many vehicles, the filter is not much more than a screen to block large pieces of debris. The screen-type filters don’t do much to prevent small abrasive wear particles from recirculating through the transmission with the fluid. Some vehicles have more efficient filters that can trap the smaller abrasive particles, but only a few (such as Saturn) have an easy-to-replace, spin-on external transmission filter.

Many vehicle manufacturers do not publish a recommended service interval for changing the transmission filter or fluid. In fact, the latest generation of ATFs are formulated to be high-mileage fluids that are capable of lasting upwards of 100,000 miles or more under normal use. The transmission is essentially a sealed assembly and the only contaminants that enter the fluid are wear particles from the clutch packs, gears and bearings inside the transmission. Even so, heat can cause varnish deposits to build up inside the transmission over time. The fluid can also burn (oxidize) if the transmission is overworked and gets too hot. That’s why vehicles that tow trailers or heavy loads typically need to have the ATF changed every 30,000 miles or so – otherwise there’s a risk of burning up the transmission!

As a rule, the transmission filter should be replaced when the fluid is changed. Many experts recommend a fluid change every 30,000 to 50,000 miles for preventive maintenance.

The best way to change transmission fluid is with a machine that performs a complete fluid exchange. Removing the pan from the bottom of the transmission to replace the filter typically drains less than half of the total fluid from the transmission. Most of the fluid remains trapped in the torque converter and can only be removed by pumping new fluid through the transmission to push out the old fluid in the torque converter.

Cabin air filters are like the filters in home furnaces and air conditioners. The filter removes dirt, dust and pollen from the air, and in vehicles it helps keep the A/C evaporator clean. This allows the A/C system to cool at peak efficiency and reduces the buildup of contaminants on the evaporator that contribute to the growth of microbes that can cause musty odors.

Some cabin air filters also remove odors, diesel fumes and other pollutants. These "combination" filters typically have a layer of activated carbon that reacts with airborne chemicals to neutralize odors. The fibers in the filter media may also be electrostatically charged to attract and hold dirt and contaminants more efficiently.Cabin air filters were introduced back in the mid-1980s in Audis and other European luxury vehicles. The first domestic applications date back to 1994 (Ford Contour and Mercury Mystique). Today, cabin air filters are found in almost 80 percent of all new vehicles.

The service life of a cabin air filter depends as much on environmental factors as mileage. Cabin air dust filters typically have a service life of two to three years, but may have to be replaced more often if the vehicle is driven on gravel roads. Cabin air odor filters, on the other hand, have a limited service life and only last about a year. After that, the activated charcoal loses much of its ability to neutralize odors.

Cabin air filters should be inspected regularly and replaced as needed – even if they are hard to reach.