Because it controls the electrical power of the pump, symptoms of a failing Fuel Pump relay often exhibit themselves as inconsistent or erratic fuel delivery. A common first symptom may be an intermittent no-start condition where the vehicle starts poorly or not at all. This symptom occurs when the relay fails to actuate and the fuel pump will not pressurize the fuel system. Most mechanics would remark that about 20% of starting issues related to fuel are actually due to a malfunction in the relay and not the pump itself.
Another symptom is that of engine stalling, mostly when either idling or driving at low speeds. A failing relay may lose contact from time to time and make the fuel pump shut off momentarily, thus disrupting fuel flow. This kind of stalling often feels abrupt, as the fuel supply to the engine stops right away, and the vehicle might restart once the relay reestablishes contact.
Another symptom of the relays is an inoperable fuel pump. This might happen if the relay is stuck in a position that keeps it in ‘off’ mode; then, even with ignition turned on, the fuel pump does not light up. Testing with a multimeter could also find out if it is receiving and sending proper voltage, typically 12 volts in most standard vehicles. A reading below that voltage suggests some internal problem in the relay.
Other symptoms of a faulty fuel pump relay are engine misfires or hesitation upon acceleration. When the relay is sending inconsistent power, it may not be able to maintain the required fuel pressure that the pump needs. This may cause the engine to run lean at times, especially when acceleration requires much fuel. This, in turn, can result in reduced power by as much as 15%, reflecting on the overall performance.
Watch out for other symptoms such as lack of noise from the fuel pump at every turn of the ignition. The pump always makes a low humming noise at start-up when pressurizing the fuel system. Such noise will not take place in case the relay has failed. All the above symptoms imply that drivers may replace the Fuel Pump relay or check it to get reliable fuel delivery again and avoid further problems with the fuel system.