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How Does an Electric Inline Fuel Pump Work?

by Don Bowman

Electric inline fuel pumps are divided into two categories: carburetor and fuel injection. Carbureted engines require low pressure — typically 10 PSI — and enough volume to maintain the fuel level in the float bowl under all engine loads. A fuel-injected engine’s demands are different. They require continuous high pressure — typically 45 PSI — at the fuel injector to maintain a uniform spray pattern regardless of the fuel volume requirement.

Inline Fuel Pump Principals of Operation

There are two principal types of mechanisms used to build suction and pressure to move the fuel. The most common is the georotor. It is the quietest and has the capability to create very high pressures. The pump mechanism consists of a stationary gear with a rotating ring.

The second is the rotory vane. In this type of pump the mechanism consists of a stationary offset rotor with sliding vanes to create the pressure. This type of pump works well, however it is noisy and less dependable over the long term.

Carbureted In Line Pump

When using an inline electric pump on an engine with a carburetor, it must be 10 PSI or less. The biggest consideration is the volume the pump is capable of producing. Thirty to 45 gallons per hour is typical of a standard pump. A fuel pressure regulator is necessary to keep the pressure in the 5-to-7 PSI range, low enough to prevent overcoming the needle’s pressure on the seat, causing the carburetor to flood. These fuel pumps are quite reliable and are used to replace the manual pump on a daily operated vehicle.

Fuel Injected Inline Pumps

The right size of electric inline fuel pump necessary for a fuel injected engine requires more consideration than does a carbureted version. The volume in gallon per hour will increase as well as the PSI it is capable of producing. Do not misinterpret the reason for the gallons per hour rating. It is not intended to depict the amount of fuel it will flow into the engine. It is used to equate the pump’s ability to maintain a constant pressure at the injector nozzle at all flow rates.

The fuel injectors do not open further as the vehicle is accelerating — the opening is fixed. However, they are opened for an ever-increasing period of time as the vehicle is accelerated. The longer the injectors are opened the higher the volume necessary to maintain the same pressure at the nozzle. The rated pump volume in gallons per hour drops dramatically when accelerating.

The next consideration in the equation is the amount of horsepower the injectors are feeding. The higher the horsepower, the larger the injector opening, resulting in a larger spray pattern. The larger the spray pattern, the larger the pressure drop at the nozzle.

All inline high-pressure pumps require a fuel pressure regulator between the engine and the pump. High pressure inline pumps feed a higher pressure continuously which is opposed by the fuel pressure regulator with no provisions for a return line. The system works well for performance purposes but falls short in terms of longevity.

The in-tank electric fuel pumps use a regulator with a return line to the tank. At an idle all the fuel over and above that needed to maintain a constant pressure on the injector is sent back to the tank. This applies much less stress on the fuel pump and lines thus giving them more durability. Some vehicle’s computers have the ability to govern the rpm of the fuel pump.

Inline Electric Fuel Pump Installation Requirements

The pump requires a solid mounting position on the frame forward of the fuel tank but as close as possible.

The pump requires a high micron filter between the tank and the pump to preclude any debris from entering the pump.

Fuel lines must be steel or aluminum of adequate dimensions befitting the application. Short fuel hose is to be used for connection purposes.

A fuel pressure regulator must be mounted between the engine and fuel pump.

A fusible link or fuse and relay must be used along with 10-gauge wire to the pump.

The pump must have a solid ground to the frame or negative terminal on the battery. The fuel pump requires 13.5 volts to operate so a good alternator and battery are necessary. Fuel pump operating efficiency drops drastically as the voltage drops.

About the Author

Don Bowman has been writing for various websites and several online magazines since 2008. He has owned an auto service facility since 1982 and has over 45 years of technical experience as a master ASE tech. Bowman has a business degree from Pennsylvania State University and was an officer in the U.S. Army (aircraft maintenance officer, pilot, six Air Medal awards, two tours Vietnam).

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