Diagnosing a fuel pump that refuses to start involves a series of steps designed to isolate the root cause of the issue. First, checking the fuel pump relay is very important because about 40% of fuel pump failures are due to electrical issues, including faulty relays. This may prevent the Fuel Pump from getting the power it needs to work. Most vehicles allow testing the relay by simply swapping it out with another relay of similar type or by using a multimeter to measure voltage supply to the Fuel Pump. The expected voltage, if all is working correctly, should be close to 12 volts when the ignition is turned to the “on” position. Lack of power may indicate either a defective relay, a defective fuse, or a fault in wiring.
Testing the fuel pressure is a very important diagnostic test. A good Fuel Pump will hold fuel pressure between 30 and 50 psi, which varies depending on the type of vehicle. Pressure gauges are available at most auto parts stores to test this. If the fuel pressure is lower than it should be, this can indicate that the Fuel Pump has failed or the fuel filter is clogged. As a fact, clogged filters are the cause of about 25% of all Fuel Pump problems.
Additionally, there is a little trick: listen for the hum of the Fuel Pump. If the ignition is turned on but the engine is not started, there should be a slight humming noise for a few seconds as the Fuel Pump works to prime the system. If this sound is not heard, the Fuel Pump may have mechanically failed.
Finally, another approach to diagnose a non-starting Fuel Pump is to check the fuel lines for any leaks or blockages. Fuel system problems are one of the causes of about 15% of the starting failure in older vehicles. Obvious leaks and disruptions to the fuel line should be repaired immediately for the appropriate functioning of the pump.
Scanning for error codes in the vehicle’s onboard computer system will also sometimes pinpoint the problem to the Fuel Pump or fuel system. As technology within the vehicles is improved, so many modern vehicles today have sensors and on-board diagnostic systems that directly monitor fuel pressure and pump operation to help point out specific faults such as a failed Fuel Pump, low voltage supply, or a clogged filter to name just a few for quicker, more precise repair.