An electric fuel pump is used on engines with fuel injection to pump fuel from the gas tank to the injectors. The pump must deliver the fuel under high pressure (typically 30 to 85 psi depending on the application) so the injectors can spray the fuel into the engine. Fuel pressure must be within specifications for the engine to run correctly. Too little pressure can starve the engine for fuel, causing it to run lean, misfire, hesitate or stall. Too much fuel pressure can cause the engine to run rough, waste fuel and pollute.
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 an electric fuel pump needs to be replaced, it can be replaced as a separate item or as a complete module 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. Roller cell pumps typically spin about 3,000 rpm. 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. A gerotor pump typically operates at around 4,000 rpm.
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 operates at higher speeds, typically up to 7,000 rpm and draws less current than older style pumps. 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 becauseo f variations in fuel pressure or flow.
When the driver turns the ignition key on, the powertrain control module (PCM) energizes a relay that supplies voltage to the fuel pump. The motor inside the pump starts to spin and runs for a few seconds to build pressure in the fuel system. A timer in the PCM limits how long the pump will run until the engine starts.
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 routes excess fuel back to the tank.
On newer vehicles with returnless EFI systems, the fuel pressure regulator is located in the fuel tank and is part of the fuel pump module. There is no fuel return line from the engine 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. The pump may run at a constant speed, or it may operate at a variable speed depending on engine load and speed. 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.
The fuel pump should last the life of the vehicle, but it may fail as a result of contamination inside the fuel tank (dirt or rust), fuel starvation (running out of gas), overheating (always driving with a low fuel level), low voltage (wiring problem), or overwork (trying to overcome a restriction caused by a plugged fuel filter). The harder a pump works, the hotter it runs and the more amps it pulls through its power circuit.
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. Also, if you smell no unburned fuel fumes at the tailpipe when cranking the engine, that would tell you the engine is not getting any fuel. 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. The problem may be a faulty fuel pressure regulator, a clogged fuel line or filter, or an electrical fault in the fuel pump wiring circuit.
Another way to tell if a no-start is due to no fuel is to spray some aerosol starting fluid into the throttle. If the engine starts, runs a few seconds, then dies, it has spark and compression but is not getting any fuel from the fuel pump.
A fuel pump can be expensive to replace. A new electric fuel pump may cost from $100 to $300 or more depending on the application, and whether you are buying just the pump or the complete fuel pump module assembly. The labor to replace a tank-mounted pump can also add $200 or more to the repair bill. So you want to make sure a bad fuel pump is the real problem and not something else before you replace the pump.