For reliable engine operation and fuel system performance, a clean fuel supply is absolutely essential. That's why replacing the fuel filter is so important. The fuel filter is the fuel system's primary line of defense against dirt, debris and small particles of rust that flake off the inside of the fuel tank. A mesh filter sock on the end of the fuel pickup tube inside the tank helps prevent the big pieces of dirt and rust from entering the fuel line, but it does not keep out smaller particles that can be so troublesome.
If not trapped by the filter, such contaminants can plug fuel metering orifices in a carburetor or prevent valves from seating. In fuel injected engines, fuel debris can clog the injector inlet screens and starve the injector for fuel. And if debris gets inside the injector, it can wear or jam the pintle valve and seat. With diesel engines, clean fuel is even more important because of the extremely close tolerances inside the injection pump.
Fuel filters on older carbureted engines typically trap particles as small as 70 to 100 microns in size (a micron is a millionth of a meter or 0.000039 inches). The diameter of a human hair is about 60 microns. By comparison, many filters for fuel injected engines trap particles as small as 10 to 40 microns in size. A good diesel fuel filter should trap particles as small as one micron in size!
The filter media may be treated paper, a blend of cellulose and synthetic fibers, glass fibers, sintered bronze, a ceramic material or even a fine nylon mesh. The barrier created by the filter media traps particles and prevents them from reaching the engine. Eventually the filter media becomes clogged with debris, which creates a restriction. If not replaced before it becomes plugged, it can cause major driveability problems (such as hard starting, lack of high speed power and/or stalling).
There are two basic types of fuel filters: "inline" filters that are mounted in the fuel line somewhere between the fuel tank and carburetor or fuel rail (EFI), and "internal" filters such as those that fit inside the carburetor fuel inlet on older vehicles.
On most fuel injected vehicles, the fuel filter will be found under the vehicle in the fuel line from the fuel tank to the engine, or on the firewall in the engine compartment. There are also some fuel filters on "returnless" EFI systems (Dodge Ram trucks and other late model Chrysler vehicles) that are located on top of the fuel tank and are part of the fuel pressure regulator assembly. Refer to your owner's manual for the exact location of the filter.
Replacing the fuel filter for preventive maintenance can reduce the risk of dirty fuel causing problems, or a plugged filter causing engine performance or starting problems. Some vehicle manufacturers say you should replace the fuel filter every 50,000 miles. Others say to replace the fuel filter only "as needed," and some make no filter replacement recommendations at all!
Many professional technicians say you should replace the fuel filter every couple of years to reduce the risk of a plugged fuel line causing driveability or starting problems.
Replacing the fuel filter can also help extend the life of the fuel pump. Why? Because on most fuel injected engines, excess fuel pressure from the fuel rail on the engine is routed back to the fuel tank through a fuel return line. If the fuel is dirty, abrasive particles can recirculate back through the fuel pump again and again accelerating pump wear and causing the fuel pump to fail prematurely. EFI fuel pumps can be VERY expensive to replace and typically cost $200 to $350 plus several hundred dollars labor because the pumps are mounted inside the fuel tank.
Replacement fuel filters typically cost less than $10 for older engines with carburetors, and $15 to $45 for most fuel injected engines, except certain Saturn models that cost $90 and are only available from Saturn dealers. The oddball Saturn fuel filter contains a fuel pressure regulator and is not available from aftermarket suppliers.
A new fuel filter should always be installed if a fuel pump is being replaced, especially if the fuel tank is found to contain rust or sediment.
On older carbureted vehicles, fuel filters are usually held in place by hose clamps on the fuel line. On vehicles with fuel injection, the fuel filter may be attached to the fuel lines with clamps or special "quick lock" couplings (which require a special tool to remove). Quick lock couplings are snap fittings that lock the filter to the fuel line internally. By inserting a special plastic tool into the fitting, the snap lock is released allowing the filter to be pulled out of the fuel line. DO NOT try to force a quick lock fitting apart and DO NOT attempt to pry it apart with a screwdriver because doing so can damage the fuel line and create a possible fuel leak.
On some import cars, the fuel lines attach to the fuel filter with banjo fittings. It is very important to replace both copper washers on the banjo fittings when the fuel filter is changed. Reusing the old washers increases the risk of a fuel leak.
WARNING: Gasoline is highly flammable! To minimize the risk of fire when replacing a fuel filter, make sure there are no sparks or open flames nearby (NO SMOKING!). Also, do not allow any fuel to make contact with a hot exhaust pipe, exhaust manifold or the catalytic converter. And if your using a trouble light with an incandescent bulb, keep it well away from the fuel lines when they are opened or trouble light will take on a whole new meaning for you. A 60 watt bulb in a trouble light gets very hot, and could easily ignite fuel that might drip or spray against the bulb. Trouble lights with cool LED lights or a fluorescent bulb would be a much safer choice for this kind of job.
CAUTION: Fuel injection fuel lines are under pressure. On some vehicles the line pressure may be as high as 85 psi or higher. The fuel pressure inside the line needs to be relieved before the fuel line is disconnected, otherwise fuel will spray everywhere.
One way to relieve pressure (or at least reduce the pressure in the fuel line), is to let the vehicle sit overnight and open the line in the morning. Be careful, though, because there will still be some residual pressure in the fuel line.
The fuel line can be depressurized by locating the schrader test valve on the fuel injector fuel rail and depressing the valve.
On EFI applications with a return line back to the fuel tank, you can locate the fuel pressure regulator on the injector fuel rail and apply vacuum to the regulator with a hand held pump. This is not possible on "returnless" EFI systems because the regulator is located inside the fuel tank near the pump.
Another option is to momentarily start the engine, then pull the fuel pump fuse or relay and let the engine run for a few seconds until it stalls from a lack of fuel.
Always wear eye protection and wrap a rag around the fuel line connection to deflect any residual pressure and fuel spray.
When you replace the fuel filter, be sure to inspect the condition of the fuel lines and hoses. Rubber hoses age harden and may develop cracks and leaks after many years of service. If a hose is brittle or is leaking, replace it without delay. The same goes for a steel fuel line that may be cracked and leaking. Fuel leaks are extremely dangerous because they can start fires.
Fuel hose for fuel injected engines has a much higher pressure rating (45 to 100 psi or higher) than fuel hose for older low pressure (4 to 10 psi) carbureted engines. The pressure rating should be printed on the hose. DO NOT use low pressure fuel hose on a fuel injected application. New clamps are also recommended if the hose has external rings clamps.
After replacing the fuel filter, start the engine and inspect the fuel filter and hoses for leaks. If the fuel filter or a hose is leaking, shut the engine off immediately and repair as needed. DO NOT drive a vehicle if it has a fuel leak, even if your fire insurance is paid up (unless you want to risk collecting on your life insurance!).