Maintain Engine Health: The Crucial Role of Fuel Filters

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Purpose

The fuel filter removes particles and contaminants from the fuel as it flows from the fuel pump to the engine.

Location

The fuel filter in a modern fuel injected gasoline powered automobile or light truck is located either in the fuel line (an in-line filter) or is located inside the fuel tank itself, as a component of the fuel pump assembly (an in-tank filter). 

In-line filters are typically located on a bracket mounted to the inner side of a frame rail, usually at the rear of the vehicle near the fuel tank or under the driver’s door. In-tank filters are part of the fuel pump module assembly. 

On older carburetor-equipped vehicles, an in-line fuel filter is located in the engine compartment and a second filter, sometimes made of sintered bronze, may be located at the fuel inlet to the carburetor. 

On diesel powered light trucks, the fuel filter is usually located in the engine compartment and filters both water and particle contaminants. Some diesel powered vehicles employ two fuel filters, one to filter water from the fuel and the other to filter particulates.

The fuel filter should not be confused with the strainer or strainer filter, which is a sock-like flexible mesh enclosing the fuel pump fuel pick-up, and designed to screen large debris from the pump.

Operation

Fuel entering the filter flows through a filter element, usually made of a pleated material, which traps particles and contaminants. In modern fuel injected vehicles, fuel filters are designed to trap all particles larger than 30 microns (0.03 millimeters).

Drivability Symptoms

A clogged fuel filter restricts the flow of gasoline to the engine. Fuel injected vehicles operate with high pressure fuel systems, so a clogged filter can result in loss of power during acceleration or when the engine is under load. 

On carbureted vehicles, a clogged filter may cause stalling at idle, stumbling during acceleration, and may prevent starting.

Inspection, Test, and Diagnosis

If a clogged fuel filter is suspected, a fuel pressure gauge can be connected to check for fuel system pressure that is below the vehicle manufacturer’s specifications. On most late model fuel injected vehicles, the gauge connects to the Schroeder valve – it resembles a bicycle tire valve - located on or near the fuel rail that feeds the fuel injectors. 

If the vehicle is not so equipped, the gauge must be connected by an adapter to the fuel line at a location specified in the vehicle manufacturer’s factory Service Manual (after releasing system pressure, as described in Service / Repair, below). Once connected, pressure can be read in most vehicles with the key on, engine off. Pressure can also be checked with the engine running.

There are no Diagnostic Trouble Codes (DTCs) specific to a clogged fuel filter. On model year 1996 and later automobiles and light trucks that are equipped with second generation On-Board Diagnostics (OBD2), DICs P0300 (random misfire), P0171, and P0174 (excessively lean air/fuel ratio, banks 1 and 2, respectively) may be caused by a clogged fuel filter, though these codes typically are caused by other malfunctions.

Reprogramming

Replacement of the fuel filter does not require reprogramming.

Repair/Replacement

Expected service life of in-tank fuel filters and many in-line filters in late model vehicles exceeds 100,000 miles. Many vehicle manufacturers recommend replacement only if the filter becomes clogged. If that occurs, the fuel tank should be cleaned before installing a new filter. 

Filter replacement in carbureted vehicles is a normal maintenance procedure. It is strongly recommended to have a fire extinguisher available. Additionally, when working on pressurized fuel systems, always wear eye protection and disconnect the negative battery terminal before disconnecting a fuel line.

Modern fuel injected vehicles retain high fuel system pressure after the engine stops. This pressure must be released before disconnecting a fuel line. 

If the vehicle is equipped with a Schroeder valve (similar to the valve on a bicycle tire) in the fuel rail, pressure may be relieved by connecting a fuel pressure gauge or using a small screwdriver to press the valve pin while covering it with a shop towel to enclose any fuel sprayed from the valve. Otherwise, the fuse for the fuel pump can be removed and the vehicle started. The engine will run briefly and then die of fuel starvation, depressurizing the system.

On many vehicles equipped with in-tank fuel filters, including many 2004 and later General Motors vehicles, the filter is a component of the fuel pump and cannot be separately replaced. 

On others, including many Toyota vehicles, the fuel filter can be separately replaced, but the fuel pump must be removed to do so. 

On Chrysler Corporation passenger cars and light trucks, the in-tank filter is replaceable after removing the fuel pressure regulator located in the fuel pump assembly. 

On many vehicles, accessing the fuel pump assembly may require removing the fuel tank. In others, including Toyota, Honda, and Nissan passenger cars, an access panel under the rear seat or in the trunk allows direct access to the fuel pump assembly and fuel filter.

An in-line fuel filter can be removed by disconnecting the fuel lines and removing the filter from any retaining bracket. A flare nut wrench, also called line wrench, should be used to loosen fuel line hex nuts to reduce the risk of rounding their corners. 

Lines that employ “quick connect” fittings – identifiable by their conically shaped metal housing – are released with a special disconnect tool, available inexpensively at most auto parts stores. (They reconnect merely by pushing together.) 

Some fuel lines can be disconnected by sliding a U-shaped “circlip” from the connector. The replacement in-line filter should be installed with the directional arrow (cast or painted on the filter body) pointing in the direction of fuel flow to the engine. Threaded fittings should not be over-tightened.

On vehicles with carbureted engines, depressurizing is not necessary and the filter can be replaced by releasing the clamps holding the rubber fuel lines to the filter.

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