The collapse of fuel pressure under idle conditions is the core mechanism of engine stalling. When the engine speed drops to 750±25 RPM, the standard Fuel supply pressure should be maintained within the range of 50-55 PSI. However, the output pressure of the Fuel Pump with deteriorated performance may suddenly drop below the 28 PSI warning line. The fault simulation of the Society of Automotive Engineers (SAE) shows that when the volumetric efficiency of the oil pump decay by more than 30%, the fluctuation range of idle oil pressure will expand from the normal ±2 PSI to ±18 PSI, causing the air-fuel ratio to instantly exceed the adjustable range of the ECU (±0.5λ value) by 400%, directly triggering the flameout protection program. The technical report on Hyundai Motor’s global recall of 1.28 million problem models in 2023 confirmed that 76% of complaints about idle stalling were due to the fuel pump output pressure being 35% lower than the designed value.
The thermal fade effect significantly increases the risk of idling. Experimental data show that if the resistance value of the copper winding of the worn pump body rises to 1.8Ω (the standard for new parts is 0.6-0.8Ω), the motor torque output decreases by 45%, and the flow attenuation rate can reach 0.8L/min·℃ when the fuel temperature exceeds 80℃ (the normal value is 0.2L/min·℃). At this time, the oil rail pressure will continue to decline under the hot idle condition, and the rate of reduction of 1.2 PSI per minute is much faster than that of the normal system (0.3 PSI/min). Bosch’s bench test proved that when the oil temperature was 95℃, the idle flow rate of the inferior pump body was only 3.2L/min (the required value was 5.5L/min), forcing the ECU to reduce the fuel injection pulse width to 1.8ms (requiring 2.5ms), and the mixed gas concentration dropped below the flammability limit of 14.5:1.
Abnormal electrical characteristics are deep warning signals. The working current was detected using A Fluke 88V multimeter. The normal idle current is approximately 4.3±0.2A. If the current fluctuation exceeds ±1A or shows periodic oscillations above 5Hz (normal fluctuation frequency ≤1Hz), it indicates poor brush contact or local short circuit in the winding. The 2024 diagnostic case library of the American Automobile Association (AAA) shows that among vehicles with idle stall, 92% have an abnormal median shift of fuel pump current exceeding ±15%, and 38% of them are accompanied by a peak current exceeding 6A (indicating mechanical jamming). At this point, the power loss reached 45W (the standard value was 32W), and the coil temperature rose at a rate of 8℃ per minute to above 125℃, triggering a thermal protective power outage.
The systematic failure cost far exceeds the early replacement cost. Continuous idling and stalling will cause the temperature of the catalytic converter to drop sharply from the normal 600℃ to 280℃, accelerate the sulfur poisoning of the precious metal coating, and reduce the purification efficiency by 60%. The additional work done by starting and stopping the motor due to a single shutdown (a 300A current impact lasting for 2 seconds) is equivalent to the wear and tear from 30 normal starts. Delphi’s economic analysis of maintenance indicates that for vehicles that ignore the initial symptoms, the total maintenance cost within two years is 2,100 (including 1,500 for the catalytic converter + 400 for the oil pump + 200 for programming), which is 42,518,700 more than the 400 for timely replacement of the pump body.