The Fuel Pump

On most modern vehicles, the fuel pump is an electric motor-driven pump located inside the fuel tank. The fuel surrounding the pump cools and lubricates it. This is why running a vehicle out of fuel repeatedly is hard on the pump — the pump runs dry without fuel to cool it and the motor overheats. A fuel pump is not cheap to replace because the entire fuel tank often has to be dropped to access it.

When you turn the ignition key to the ON position — before cranking — you can hear a brief hum from the rear of the vehicle for about two seconds. That is the fuel pump pressurizing the system. The PCM commands the fuel pump relay on for a short prime cycle to build pressure before cranking. Once the engine starts and the PCM receives a valid crankshaft position signal, the pump runs continuously. If the engine stalls and the PCM loses the crank signal, it shuts off the fuel pump relay within about one second. This prevents the pump from running with the engine off — a fire safety feature.

The fuel pump relay is the gatekeeper. It is a small electromagnetic switch in the fuse box that the PCM controls. Battery power feeds one side of the relay contacts. When the PCM provides a ground to the relay coil, the contacts close and power flows to the fuel pump. If you have a no-start and cannot hear the pump prime, do not assume the pump is dead. Swap the fuel pump relay with an identical relay from another circuit in the same fuse box — like the horn relay or AC relay if it is the same part number. If the pump runs with the swapped relay, the relay was the fault. This is a two-minute test that saves you from dropping a fuel tank unnecessarily.

Long crank before starting — the pump is weak and takes longer to build adequate pressure. Loss of power under heavy acceleration or at highway speed — the pump cannot maintain pressure under high demand. Engine stalls intermittently and restarts after sitting — the pump motor is failing intermittently due to worn brushes or a failing commutator. Complete no-start with no fuel pressure at the rail — the pump has failed completely. Whining or buzzing noise from the rear of the vehicle that gets louder over time — the pump motor bearings are wearing out.

Always check the fuel pump relay, the fuel pump fuse, and the wiring and ground connections first. Test for battery voltage at the fuel pump connector with the key on. If you have voltage at the connector but the pump does not run — the pump is bad. If you have no voltage — the problem is upstream. Check the relay, fuse, inertia switch if equipped, and all wiring. A bad ground or a corroded connector at the pump causes the same symptoms as a failed pump at a fraction of the cost. Also check that the fuel pump control module is commanding the pump — on returnless systems, a failed control module can prevent pump operation.