Electric fuel pumps are usually mounted inside the fuel tank, though some may be mounted outside the tank. Some vehicles may even have two fuel pumps (a transfer pump inside the tank, and a main fuel pump outside). The in-tank location helps muffle the buzzing noise produced by the electric pump motor, and immersing the pump in fuel helps lubricate and cool the pump motor. Driving with the fuel tank less than 1/4 full can shorten pump life by causing it to run hot. It also increases the risk of momentarily starving the pump for fuel when cornering sharply, braking or accelerating. Running out of gas can sometimes damage an electric fuel pump by starving it for cooling and lubrication.

The pump is usually part of the sending unit assembly, that includes a float that sends an electrical signal to the fuel gauge on the instrument panel. If the fuel pump needs to be replaced, it can be replaced as a separate item or as a complete assembly (which is more expensive but easier and less troublesome).

Electric fuel pumps come in a variety of designs. Some older applications use a positive displacement "roller cell" pump. This type uses rollers mounted on an offset disc that rotates inside a steel ring. Fuel is drawn into the spaces (cells) between the rollers and pushed along from the pump inlet to the outlet. This type of pump can generate very high pressure, and the flow rate tends to be constant. But the output comes in pulses, so a muffler is often mounted in the fuel line after the pump to dampen pressure pulses. A roller cell pump may also be mounted outside the fuel tank, and used with a second low pressure supply pump mounted inside the fuel tank.

Another type of positive-displacement pump is the "gerotor" pump. This design is similar to that of an oil pump, and uses an offset rotor to push fuel through the pump.

Another variation is the roller vane pump. Here, vanes are used instead of rollers to push fuel through the pump.

Most newer vehicles use a "turbine" style fuel pump. A turbine pump has an impeller ring attached to the motor. The blades in the impeller push the fuel through the pump as the impeller spins. This type of pump is not a positive-displacement pump, so it produces no pulsations, runs very smoothly and quietly. It is also less complicated to manufacture and is very durable. Some aftermarket pump supplies use this type of pump to replace the older designs.

NOTE: Replacement fuel pumps do NOT have to be the exact same type as the original. But they must be capable of generating the same operating pressure and delivering the same volume of fuel as the original. Using the wrong pump or substituting a different pump can cause drivability problems.

How an Electric Fuel Pump Works

Fuel is drawn into the pump through an inlet tube and mesh filter sock (which helps keep rust and dirt out of the pump). The fuel then exits the pump through a one-way check valve (which maintains residual pressure in the system when the pump is not running), and is pushed toward the engine through the fuel line and filter.

The fuel filter traps any rust, dirt or other solid contaminants that may have passed through the pump to prevent such particles from clogging the fuel injectors.

The fuel then flows to the fuel supply rail on the engine and is routed to the individual fuel injectors. A fuel pressure regulator on the fuel rail maintains fuel pressure, and recirculates excess fuel back to the tank.

The fuel pump runs continuously once the engine starts, and continues to run as long as the engine is running and the ignition key is on. If the engine stalls, the PCM will detect the loss of the RPM signal and turn the pump off.

Many vehicles (Fords, notably) also have an "inertia safety switch" that shuts off the fuel pump in the event of an accident. This is done to reduce the risk of fire should a fuel line be ruptured. A hard jolt trips the safety switch and opens the fuel pump circuit. This required manually resetting the safety switch after the incident by pressing the reset button on the switch.

On most older vehicles, the fuel pump runs at a constant speed. But on many newer applications, the pump speed is varied by the PCM to more closely match the engine's fuel requirements.

Fuel Pump Failure

When a fuel pump fails, it often just quits without any warning. You are driving along fine one minute, then suddenly your engine stalls and you are stranded alongside the road. Or, you come out to start your car in the morning only to discover it cranks but won't start.

How can you tell if a bad fuel pump is causing your no-start problem? One way is to listen for pump noise after turning the ignition key on. No pump noise would tell you the pump is not running. The fault may be a bad pump, or it could be a bad fuel pump relay, fuse or wiring connection.

On most vehicles, a failed fuel pump will not set any diagnostic trouble codes or turn on the Check Engine light (Malfunction Indicator Lamp). The engine will crank normally, and it will have spark, but it will not start because it is not getting any fuel.

Most late model engines have a fuel pressure test fitting on the engine fuel rail. Attaching a fuel gauge to the schreader valve fitting will quickly reveal whether or not the pump is generating any fuel pressure. On engines that do not have a fuel pressure test fitting, a fuel pressure gauge can be teed into the fuel line where it connects to the fuel rail. If fuel pressure is zero, the pump is not working. If fuel pressure is less than specifications, further diagnosis will be needed to determine why.