Regular filter inspections and replacements are necessary to protect the engine, transmission and fuel system against external and internal contaminants. The factory recommended service intervals (or lack thereof) for many filters are usually mileage-based, and may not take into account how a vehicle is driven or environmental factors.
Extended service intervals are based on using high-quality filters and fluids as well as “normal” driving. Although most motorists think they are “normal” drivers, the truth is many are actually “severe service” drivers who should follow the more frequent severe service recommendations in their owner’s manual.
Factory recommendations for replacing an air filter may range from 30,000 miles up to 50,000 or 60,000 miles. A few, such as certain Ford Focus PZEV models, even have a “lifetime” (100,00 mile-plus) air filter. Even so, air filters should always be inspected when the oil is changed and replaced as needed regardless of mileage.
Many consumers don’t know how to inspect a flat panel air filter. If you open the air filter housing, what you see if the “clean” side of the filter. The dirty side is underneath. The filter should be removed from the housing and held up to a shop light for inspection. The less transparent the filter is, the dirtier it is and the sooner it will have to be replaced.
A dirty air filter restricts airflow into the engine. The Powertrain Control Module (PCM) can compensate for this up to a point by readjusting the fuel mixture. But eventually fuel economy and performance will suffer if the filter is not replaced.
Cabin air filters prevent dust and odors from entering the passenger compartment. Dust-only filters should be replaced every 20,000 to 30,000 miles, while dust/odor combination filters usually need to be changed yearly or every 15,000 miles. The filter is usually located behind the glovebox or in the cowl area at the base of the windshield. The filter is often neglected because many motorists don’t know their vehicle has one.
Oil filters ensure a clean oil supply to the engine’s bearings. The oil filter should be replaced every time the oil is changed. Recommended service intervals on late-model cars can vary from 5,000 to 7,500 miles, or up to 10,000 miles or more. Some vehicles use an oil reminder system rather than a simple mileage chart to signal when an oil change is due. For extended oil change intervals, it’s important to use a premium-quality, long-life oil filter and synthetic oil, not the cheapest filter on the shelf and conventional motor oil.
Fuel filters protect the fuel injectors, not the fuel pump. The pump pulls unfiltered fuel from the tank and pushes it to the injectors. The pump’s only protection (which is minimal) is the screen on the pump inlet tube.
Fuel-injected engines require very high efficiency filters that can trap debris as small as 10 to 25 microns in size. On diesel engines, injector tolerances are even tighter so diesel fuel filters trap particles that even smaller.
Service intervals for “in-line” filters between the fuel tank and injectors may range from 30,000 to 50,000 miles. Most in-tank filters are “lifetime” filters with no service interval, and are only replaced if they have plugged up or a new fuel pump is being installed.
In-line fuel filters are directional with one port marked “in” and the other marked “out,” or an arrow showing the direction the fuel flows. If the filter is installed backwards, the filter’s internal bypass valve won’t work properly and the media may be damaged.
Automatic transmission filters are another “lifetime” filter that is seldom replaced unless a vehicle is having transmission problems or the transmission fluid is being changed. The filter is usually located inside the pan on the bottom of the transmission. Replacing it requires a new pan gasket and ATF.