A Fuel Pump is critical to idle quality because it is supposed to supply fuel at the right pressure constantly to the engine. Most modern engines require fuel pressure to be maintained within a narrow bracket between 30 and 80 psi for optimal performance, which can easily be compromised when the Fuel Pump wears out or becomes weak. This could further lead to poor fuel delivery, showing itself as poor engine idling. When fuel pressure is low, the engine might run lean-so to say-rough idle, misfire, and sometimes stall.
For example, whenever there is a blockage in the fuel filter, the Fuel Pump will always have to struggle much, hence reducing efficiency and lowering the fuel pressure. In fact, it has been determined that a loss in pressure of only 10 psi can cause serious imbalance in the air-fuel mixture, leading to incomplete combustion-a factor which directly influences idle quality. Similarly, if the Fuel Pump is defective or is well along its life of 100,000 to 200,000 miles, it may indeed not be able to hold the required amount of fuel, thus causing a bad idle.
Especially at idle, when fuel requirement is relatively low, car engines need a steady and precise fuel supply to keep the right mixture of fuel and air. The symptoms of a weak Fuel Pump are surging-when the engine revs up and down erratically. Since this is usually brought about by the irregular fuel delivery causing the engine RPMs to fluctuate, this normally happens at about 600 to 1000 RPM during idle. Such surging could affect not only idle quality but fuel economy, possibly even leading to damage to the engine over time.
Fuel Pump failures that caused engines to stall or fail to idle correctly were the basis of a 2019 Subaru recall of over 500,000 vehicles. Rough idle and stalling in those affected were symptoms of inadequate delivery due to a failed Fuel Pump, showing the possible effect of a Fuel Pump on idle quality.
As Henry Ford once said, "Quality means doing it right when no one is looking." For the Fuel Pump to do the job right, fuel pressure should be routinely checked and worn parts replaced to maintain a smooth idle and ensure overall engine performance.
For a better understanding of how the Fuel Pump affects the quality of an idle and how to maintain the fuel system in good health, please refer to the article entitled Fuel Pump.