The recommended replacement intervals for fuel filters vary, depending on the type used. If a filter is installed in a fuel line, it's referred to as "external"; the "internal" type is part of the fuel pump system and is located inside the fuel tank. In-tank filters, or "socks," are usually considered a "lifetime" part.

Most carmakers recommend replacing the external fuel filter at 30,000 miles for "normal service," and at 12,000 to 15,000 miles under "severe service." Check the owner's manual for the proper replacement intervals. Also inspect the filter hose for exterior deterioration from road film, oil and nicks, etc., and replace it as necessary.

Note that fuel systems without a return, only have an internal in-tank filter. These all-in-one pump/filter/sending units are costly to replace, but must be serviced if fuel delivery is restricted and engine performance is affected. Inspect all fuel lines for signs of damage, and look for cracks or crimping under the hose clamps.

ATF Filter
Transmission filters are the usually forgotten items when it comes to preventive maintenance. In most cases, they're not as easy to replace as oil, air or fuel filters, so unless there's reason to believe that a plugged filter is affecting transmission performance, we tend to overlook them. In fact, though, the ATF filter is protecting the second most expensive component on the vehicle, and should be cleaned or replaced at the manufacturer's suggested interval.

Automatic transmissions use either a replaceable paper or felt filter or a screen to remove contaminants from the fluid.

Several automakers, including GM, have introduced "long-life" or "maintenance-free" transmissions. Many of these boxes do not use a dipstick and have no requirement to service the fluid or filter for 100,000 miles under normal service. Of course, "normal service" is the catch, because some manufacturers specify a fluid and filter change at 50,000 miles for severe service (short-trip, stop & go driving, trailer towing, etc.).

Air Filter & PCV Valve
The air filter and PCV valve are often neglected maintenance items, and may not be replaced until they're so plugged up that engine performance suffers. It's important to inspect and replace them regularly to prevent premature engine wear or damage and to maintain maximum engine efficiency and performance.

The protective value of an air filter cannot be overstated. The air filter removes dirt, carbon and other contaminants from the incoming air before it mixes with fuel. As much as 10,000 gallons of air may pass through an air filter for every gallon of fuel the engine burns, and even the tiniest contaminants can cause serious damage to internal engine components.

Generally, most manufacturers recommend air filter replacement at 30,000 miles and inspection at 15,000 miles under "normal service." Operation under "severe service" conditions calls for replacement at 15,000 miles.

It's good to get into the habit of inspecting the air filter during routine oil changes. Remove the filter and closely check the areas deep inside the pleats. Filters that look clean on the surface may be extremely dirty inside and require replacement.

The PCV system routes blowby gases from the crankcase into the induction system, where they're reburned in the combustion process. Over time, oil, gasoline and condensation build up as sludge inside the PCV valve, plugging up the valve and increasing crankcase pressure. This can force oil through seals and gaskets, leading to excessive oil consumption and leakage. Increased crankcase pressure can also force PCV vapors through the breather element, depositing oil and hydrocarbon particles on the air filter itself. When a sticky PCV valve doesn't fully close, false air is introduced into the combustion chamber, diluting the air/fuel mix. The result can be poor idle quality and hesitation.

The service interval can vary widely between manufacturers, but every 30,000 miles is a good rule of thumb for normal service. When in doubt, consult the owner's manual or a shop manual.