Excessive flow rate deviation directly leads to overfuel supply. Bosch Laboratory’s sampling test of the aftermarket shows that 17% of non-original fuel pumps have a positive flow deviation of more than 8% (ISO 16380 standard allows ±4%). A typical case is the Delphi FE0113 pump, which has a nominal flow rate of 300L/h and is measured to reach 326L/h. This results in the fuel injection pulse width remaining constant, causing the fuel-air ratio to deteriorate from 14.7:1 to 12.1:1. The failure statistics of the Society of Automotive Engineers (SAE) in North America indicate that such situations account for 63% of cases of overly rich mixtures after replacement.
The loss of control of pressure forced the ECU to be passively calibrated. Modern direct injection systems require rail pressure control accuracy of ≤±15bar (BMW N55 specification), but the spring stiffness deviation of the pressure reducing valve of inferior fuel pumps often reaches 30%. The actual measurement shows that when the pressure reduction drops from 4.0bar to 3.1bar, the fuel reflux rate on the low-pressure side increases sharply by 40%, triggering the ECU to compensate by increasing the stroke of the high-pressure pump, ultimately increasing the fuel injection volume by 22%. Volkswagen Group’s technical service notification (TSI-19-01) confirmed that this cause accounted for 37% of the mixed gas faults in the EA888 engine.
Electrical noise interferes with the sensor signal. The electromagnetic interference (EMI) intensity generated by the electric arc of the low-quality pump brush in the 200-1000 MHZ range reaches 85dBμV, which exceeds the immunity standard of the vehicle ECU by 34%. Toyota’s actual measurement shows that this will distort the current waveform of the fuel injector, causing a random increase of 7% to 18% in the single injection volume. In Mazda’s 2022 recall (REV: 0022J), an electromagnetic compatibility issue led to a false alarm from the oxygen sensor that the air-fuel ratio was too thin, causing the ECU to increase its concentration by 12% in reverse.
Inaccurate installation matching disrupts the dynamic balance. When the output impedance of the new pump is reduced by more than 0.8Ω compared to the original factory (commonly seen in large-flow modified pumps), the duty cycle control curve of the drive module fails. The Audi S4 case shows that after replacing the 380L/h high-flow pump, the fuel supply system experienced a 22% flow overshoot at a sudden load of 3000rpm, and the instantaneous air-fuel ratio dropped to 11.3:1. The trigger probability of this phenomenon is as high as 91% when accelerating rapidly with a 30% throttle opening.
The solution relies on system-level calibration. The Ford SSM 50132 technical bulletin requires that after replacing the Fuel Pump, the fuel transfer function must be rewritten using the IDS device, with an average of 173 calibration parameters. Mercedes-benz’s high-precision tests show that, in combination with the closed-loop learning of the air-fuel ratio sensor, the deviation of the concentrated mixture can be controlled within ±1.2%. For users, the original factory spare parts (cost 120-280) are 57% more expensive than the aftermarket parts (45-90), but the standard deviation of the flow accuracy is less than 0.8L/h (the aftermarket parts reach 4.2L/h), which can avoid potential losses such as oxygen sensor poisoning and three-way catalytic converter blockage caused by concentrated mixed gas from the root (the single maintenance budget exceeds $850).