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FILTERS: Motorists May Ignore Them, But They Can't Afford To

By Larry Carley


When the economy slumps, what’s the first thing many motorists cut back on to save money? Maintenance. Repairs can’t be postponed because if their vehicle breaks down, it has to be fixed. But maintenance is something that’s performed at the vehicle owner’s discretion. He or she can decide what to service and when depending on when it’s convenient or when they can afford it.


One of the most common misconceptions about filter maintenance is that aftermarket recommendations are typically more often than necessary. The recommended filter replacement intervals by aftermarket suppliers are not necessarily motivated by a desire to sell more product, but are based on intervals that better suit the typical driver than those recommended by the vehicle manufacturers in their owner’s manuals.


OEM filter replacement intervals are usually based on “ideal” operating conditions, which means mostly highway driving, trips that are at least six or more miles in duration, limited idling and not a lot of stop-and-go city driving, infrequent short trips (especially during cold weather), little or no driving on dusty gravel roads, and no trailer towing. The OEM filter replacement intervals are also designed to minimize maintenance expense for the vehicle owner. That looks good on paper, but in the real world it can sometimes result in expensive engine wear and costly repairs.

Take oil and filter replacement intervals, for example.


The aftermarket has long recommended changing both every 3,000 miles. Yet many OEMs say changing the oil and filter is only necessary every 7,500 miles. Those that use oil reminder service lights to signal when an oil change is needed may not turn on the light until the oil has racked up 9,000 to 12,000 or more miles. The mathematical models that are used to predict oil life are based on new engines in a test lab, not everyday driving under a variety of operating conditions. Nor do most of the models compensate for increased engine wear and blowby that occurs as the engine accumulates miles. Consequently, the extended oil change interval that seems adequate when an engine is new may no longer be adequate once the engine has 75,000 or more miles on it.


Oil and filter change intervals can also vary depending on the type of oil used (synthetic or conventional), the additive package in the oil itself, and the type of filter used (standard or premium).


The point here is that you need to stress the importance of regular filter and fluid maintenance to your customers. Filter maintenance is something that should NOT be postponed. Ignore the filters and eventually you’ll pay the piper. 

If a motorist puts off changing his oil and filter too long, he increases the risk of engine sludge causing expensive damage.


If a dirty air filter isn’t changed, it can hurt fuel economy and performance.

If the fuel filter is never changed, it may plug and leave the motorist stranded.

If the transmission filter and fluid are ignored long enough, the transmission may experience shift problems and wear that can result in expensive repairs.

If the cabin air filter is never replaced, the output of the heater, defrosters and air conditioner may be restricted.



Selling oil and air filters today is a little more complicated than it used to be due to a proliferation of products from filter suppliers. Most filter suppliers no longer offer a single oil filter or air filter for a given application. They now have two or in some cases three different grades of filters for the same application. Typically, the offer a “standard” OEM-equivalent grade of replacement filter, plus some type of upgrade or premium replacement filter for a couple of bucks more.


Standard grade filters are targeted at customers who don’t want to spend any more than is absolutely necessary on maintenance. Brand name standard grade filters also have to be price competitive with no-name, private label and imported filters, some of which may not be the best quality. Consequently, the price difference can’t be too great between the brand name and the no-name filter.


Premium grade filters, which include higher-efficiency or extended-life filters, cost a few bucks more but have real advantages over standard filters. These types of filters are designed for the vehicle owner who wants the best protection for his engine. He doesn’t mind spending a little more for a really good filter that will do a better job of keeping his engine clean.


There are also performance air filters that appeal to motorists who want a lower restriction filter that helps wide-open throttle engine performance. These filters often use an oil-wetted cotton fiber filter media to reduce air restriction. Such filters are also washable to extend their service life.



The typical OEM or standard replacement oil filter has an efficiency rating of around 97.5 percent. That means it will trap 97.5 percent of the particles that pass through the filter. A standard oil filter will usually trap most of the dirt and wear particles 15 to 40 microns in size and larger (a human hair is about 80 microns in diameter). For smaller particles, the efficiency drops because the particles slip through the media without being snagged.

With premium filters, a blend of fibers is often used to trap the smaller filters and improve efficiency to as high as 99.5 percent or better. Synthetic microfibers are blended with cellulose filters to form a spider web maze that stops down to 1 micron in size or even smaller.


As contaminants build up inside an oil filter (or any type of filter for that matter), the filter becomes more efficient at trapping even more particles. But too much debris can restrict flow through the filter. And if the filter becomes too restrictive, it can force the bypass valve to open allowing unfiltered oil to circulate through the engine. This can cause accelerated engine wear and premature engine failure. 


Oil filters should be replaced with every oil change, especially if the oil is not being changed regularly (every 3,000 miles). If a low-mileage vehicle is being used mostly for highway driving, and the owner is changing the oil every 3,000 to 5,000 miles, he could probably get away with changing the filter at every other oil change to save a few bucks. But as cheap as oil filters are, why take the risk? Besides, why contaminate the new oil with a pint or more of old dirty oil?


Many oil filters are not easy to replace, thus there is a tendency to neglect them. A filter wrench may be needed to reach a buried filter. Filters that have a non-slip coating improve your grip and make filter replacement much easier.



The air filter is the engine’s first line of defense against external contaminants. The filter prevents airborne silica and dust from being sucked into the intake system. This protects the airflow sensor from being coated with gunk that could upset the air/fuel mixture and cause emissions, fuel economy and performance problems. The filter also stops particles that could cause engine-damaging wear to the piston rings, cylinders and bearings.


Most air filters are made of resin treated cellulose (paper) combined with synthetic fibers. Most standard replacement air filters have an efficiency rating of around 96.5 percent, and will trap most dirt particles that are 5 to 6 microns in size and larger, along with 80 to 90 percent of the particles down to a couple microns in size. Some of the best premium air filters are rated at 99.5 percent efficiency or higher, and are very good at trapping particles down to 1 micron size.


The OEM recommended replacement interval for many air filters today ranges from 30,000 to 60,000 miles, but these intervals are for ideal operating conditions. A more realistic service interval is to inspect the filter yearly, and replace it every 12 to 24 months, or every 12,000 to 24,000 miles depending on driving conditions.



They call it the “hidden filter” because many motorists don’t realize their vehicle has a separate air filter for the passenger compartment. Cabin air filters first appeared back in the mid-1980s. The earliest applications were on Audi and other European makes. Today, most new import and domestic vehicles have a cabin air filter — or a slot where one can be installed. 


Cabin air filters are put there for the health of the vehicle’s occupants. The filter can trap pollen, dust, smoke and other pollutants that would otherwise enter the vehicle and possibly irritate the nose and lungs of the driver and passengers.


Most of these filters are highly efficient and have electrostatically charged fibers that do an excellent job of trapping even the smallest particles (down to 0.3 microns!). Most cabin air filters will stop 100 percent of all particles that are 3 microns or larger in size, and 95 to 99 percent of particles in the 1- to 3-micron-size range.


Some cabin air filters also trap odors, and are called “combination” filters. These filters have an extra layer of activated carbon that reacts with odors and other airborne pollutants to neutralize them before they become objectionable. The filters can even reduce the levels of carbon monoxide and oxides of nitrogen from the exhaust of other vehicles. The levels of these pollutants can be quite high in heavy stop-and-go traffic, and it’s not unusual for the concentration of these pollutants to be several times higher inside a vehicle than outside. Studies have shown that driver reaction times are slower when the driver is being affected by poor air quality.


Some cabin air filters are also treated with a chemical biocide or a special surface treatment that destroys bacteria, fungus and mold spores on contact. This is important to prevent the growth of unwanted organisms and odors.

The recommended replacement interval for a cabin air filter depends on the type of filter (pleated paper or a combination filter with activated carbon), and the filter’s exposure to environmental pollutants. 

 As a general rule, most cabin air filters should be changed every 20,000 to 30,000 miles — or more often depending on the size and capacity of the filter. Some vehicle manufacturers recommend replacing an odor-absorbing cabin air filter every 12,000 to 15,000 miles or once a year to keep the filter working at peak efficiency.



Fuel filters used to be a regular maintenance item, but not so anymore. More and more new vehicles are being factory-equipped with “lifetime” fuel filters that are located inside the fuel tank and are part of the fuel pump module. These filters do not have a replacement interval and should not have to be replaced unless the fuel tank has become contaminated or the fuel pump is being replaced.


For older vehicles with filters mounted somewhere in the fuel line between the tank and engine, changing the filter for preventive maintenance can reduce the risk of a breakdown or no-start due to a clogged filter. Regular filter changes can also prevent contaminants in the fuel system from plugging up the fuel injectors.


Fuel filters on most fuel injected engine (except those with “returnless” EFI systems) flow a lot more fuel than the engine actually burns. That’s because the fuel continually recirculates from the engine back to the tank when the engine is running. So the same fuel is actually filtered over and over again every time it makes the loop from the tank to the engine and back.


 Most fuel filters will trap particles 3 microns or larger in size, and some will trap particles that are even smaller. And just like air and oil filters, filtering efficiency goes up the dirtier the filter gets. But at the same time, so does the resistance to flow.


A plugged fuel filter is bad news because it causes a drop in fuel flow that chokes off the engine’s fuel supply. A dirty filter may flow enough fuel at idle and low rpm not to cause any noticeable problems, but it may not pass enough fuel at higher engine speeds causing a loss of power and hesitation. If the filter plugs up, it can cause the engine to stall or make it hard to start. Many fuel pumps have been replaced unnecessarily because the real problem was a plugged fuel filter, not a bad pump.


Many experts recommend replacing the fuel filter for preventive maintenance every two to three years or 30,000 miles, or anytime a fuel restriction is suspected. The fuel filter should also be replaced if the fuel pump has failed and is being replaced.



Many motorists don’t realize their transmission has a filter. On a few vehicles, the manufacturer made filter changes easy by using a simple spin-on external filter like an oil filter (Saturn, for example). But on most vehicles, the transmission filter is sealed up inside the transmission pan, which requires a lot of messy disassembly to replace it.

The transmission filter keeps the transmission fluid clean to minimize wear and the risk of control solenoids jamming or sticking. The filter has a somewhat easier job than that of an oil filter because there’s no combustion blowby, moisture and soot contamination in the oil. But the internal clutches produce wear particles that can cause trouble if not filtered out.


On most late model vehicles, there is no recommended replacement interval for the transmission filter. Or, it may be 100,000 miles (which is never to many motorists). Even so, most transmission experts will tell you changing the transmission fluid and filter every 50,000 miles can significantly reduce the risk of transmission-related problems and premature failure.


Note: If a customer is buying a transmission filter, he will also need new pan gaskets for the transmission when he changes the filter.  

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