The operation of the Fuel Pump is directly related to engine idle stability. When the fuel pressure drops below the originally factory-set value (such as 50 psi in a 2.5L Toyota Camry) by more than 10%, the AFR time-space fluctuation range may rise from ±0.3 to ±1.5, which would cause rotational speed oscillations (±150 RPM vs. ±50 RPM). For instance, Honda recalled 158,000 cars in 2021 due to a fuel pump pressure sensor fault. The idle stalling rate of the faulty cars was up to 12%, but upon replacement, the fluctuation range came down by 80% (data source: NHTSA Recall report). When Fuel Pump flow rate degrades to 75% of its rated capacity (for example, from 5 L/min to 3.75 L/min), the injection error rate of the fuel injector increases from ±1.5% to ±5%, and the likelihood of erratic idle speed fluctuations (600-900 RPM) increases to 25% (Source of data: SAE 2022 study).
Clogging due to impurities and wear increase abnormal idle speed. When the clogging rate of the Fuel filter is above 30% (pore size < 35 microns), the Variance of the output pressure of the Fuel Pump is enlarged from 0.5 to 2.0. The ECU has to lengthen the fuel injection pulse width (from 2ms to 3ms), but the atomization particle size of the fuel increases from 20 microns to 50 microns, and the combustion efficiency will be decreased by 12%. Based on J.D. Power’s data in 2023, the complaint frequency regarding unstable idle speed due to pump body corrosion of vehicles running on ethanol fuel (E15) is 2.3 times higher than that of regular gasoline vehicles, and the repair cost rises from $300 to $800 (fuel injector replacement included). Such an example is when the user’s feedback on Ford EcoBoost 1.5T engine tells us that once the fuel pump filter screen gets plugged, then the cylinder pressure oscillates ±8 psi while idling speed (normal ±2 psi) and the rate of misfire increases from 0.1 times/min up to 4.5 times/min.
Periodic idling problems are attributed to electrical anomalies. If the carbon brushes of Fuel Pump motor wear out (life less than 100,000 start-stop cycles), the value of resistance is raised from 2Ω to 6Ω, and the range of fluctuation of working current is raised from ±0.5A to ±2A, the ECU interprets the fuel pressure incorrectly and orders idle speed correction (±200 RPM). Volkswagen Group’s Technical Bulletin 2022 discloses that poor contact in the fuel pump circuit of MQB platform vehicles causes the median idle speed move from 750 RPM to 850 RPM, and there are chances that the STFT value crosses the limit (±15%) with an 18% probability. In addition, oxidation of fuel pump relay contacts (resistance > 0.1Ω) will cause the supply voltage to drop to 9V (normal 12-14V), resulting in a 20% flow decrease and an increase in idle jitter amplitude from 0.5mm/s² to 2.0mm/s².
Idle performance is temperature- and load-dependent. Under a high-temperature condition (> 40°C), the insulation resistance of Fuel Pump motor winding decreased from 10MΩ to 1MΩ, the power consumption increased from 5A to 8A, the frequency of occurrences of fuel Vapor Lock increased by 15%, and the possibility of stalling due to the pressure drop at idle (from 50 psi to 30 psi) increased to 8%. General Motors’ high-temperature tests in 2023 showed that fuel pumps lacking ceramic heat-dissipation coatings had a flow rate reduction as high as 12% when idling at high temperatures, while the advanced design version sustained only 2%. In low-temperature cold starts (< -20°C), the viscosity of the fuel increases from 3 cSt to 40 cSt, Cavitation in the pump body (Cavitation) reduces the flow by 30%, idle speed needs to be increased to 1200 RPM in order to ensure stability, and fuel consumption is 18% higher (Data source: SAE Cold Start Study).
Sensor and calibration faults aggravate idle speed problems. When integrated fuel pressure sensor accuracy deviation (e.g., Bosch EVO-6) exceeds ±2%, the delay of the ECU feedback control increases from 10ms to 50ms, and idle speed periodically fluctuates (±200 RPM). BMW B48 engine user experience shows that when the sensor failed, idle AFR ranged from 14.7:1 to 13.5:1 or 16.0:1, and the likelihood of CO emissions exceeding the norm increased to 34%. In addition to that, if the modifying market’s fuel pump is not programmed to the ECU (i.e., in this example, increasing the 20% rise in flow rate but not rewriting the fuel mapping), it will produce too-rich an idle mixture (λ value 0.8-0.9), a 50% increase in hydrocarbon (HC) exhaust, and a 300% increase in carbon deposit rate for the spark plugs.
Preventive maintenance completely transforms idle quality. Replacement of the fuel filter screen at 30,000-kilometer intervals ($30) can reduce the clogging frequency of the pump body by 72% and decrease the idle pressure fluctuation range to ±3 psi (down from ±10 psi). Illustrate with the use of the Nissan Altima. After maintenance, idle speed standard deviation (SD) decreased from 25 RPM to 8 RPM, and the expense of annual maintenance was saved by approximately 150 US dollars. As per industry statistics, out of worldwide idle failure cases in 2023, fuel pump related cases were 29%, and 85% of them were able to avoid major engine overhauls by early diagnosis (data source: Frost & Sullivan analysis).
