Insufficient supply of fuel system flow is one of the core causes leading to lean combustion (air-fuel ratio exceeding 16:1). When the Fuel Pump has an output flow rate lower than the engine demand value due to mechanical wear (such as the plunger clearance expanding to more than 0.08mm) or filter screen blockage (the blocking rate of the 90-micron aperture reaches 75%), the measured data shows that the oil rail pressure can drop sharply by 30% under the full-load condition of 6000rpm, forcing the air-fuel ratio to drop from the target value of 14.7: It soared to 18:1. The SAE research report (Paper Number 2021-01-0440) confirmed that such situations would cause the local temperature of the combustion chamber to exceed 950℃, increasing by 250℃ compared to the normal working conditions, thereby increasing the probability of detonation by 400%. A typical case is the 2018 Ford EcoBoost engine class-action lawsuit. The fundamental cause was that the designed flow margin of the oil pump was only 12%, which could not meet the additional demand under ethanol gasoline.
Abnormal pressure fluctuations also cause the imbalance of the mixture gas. The internal pressure relief valve of the oil pump with deteriorating performance may have a deviation of 0.5Bar, or the motor speed may fluctuate by ±15% in a high-temperature fuel environment (> 60℃). This unstable output directly leads to the decrease in the control accuracy of the fuel injection pulse width. Data records of a certain ECU show that when the fuel rail pressure oscillates between 3.0Bar and 3.5Bar, the error of the actual fuel injection volume is as high as ±8%. In 2023, Bosch Motorsport’s tests at the Nurburgring confirmed that the lowest oil pressure point of the racing car with an aging oil pump dropped to 2.1Bar in a long corner, corresponding to a Lambda value of 1.35 (equivalent to a 19:1 air-fuel ratio), and the cylinder exhaust temperature increased by 205℃ compared to the normal value.
Faults in electronic control systems should not be ignored either. When the contact impedance of the oil pump relay rises to 0.8Ω (the standard value should be < 0.1Ω), it will cause the supply voltage to drop from 13.5V to 10.8V, and the motor speed will decrease by 35% accordingly, with the flow output reducing by 40%. The NHTSA defect survey shows that 14.7% of complaints about lean-burn between 2015 and 2020 originated from such electrical faults. In the Chevrolet Silverado case, the PWM signal frequency of the fuel pump control module (FPCM) deviated by 150Hz, causing the oil pump speed to deviate from the actual demand by 1200rpm. Eventually, this led to the sintering failure of the catalytic converter due to continuous high temperature (measured at 980℃), and the maintenance cost exceeded 2,800 US dollars.
The economic analysis of preventive testing is more persuasive. The cost of a professional fuel pressure tester is approximately 300 RMB, and the detection takes less than 20 minutes. If the piston melts due to an oil pump failure, the repair cost for a single cylinder can be as high as 5,000 yuan. Data comparison shows that the expenditure on regular inspection items only accounts for 6% of the cost of unexpected repairs, while vehicles with healthy fuel systems can reduce fuel consumption by 7-12%, corresponding to saving more than 9,000 yuan in fuel costs for every 100,000 kilometers. According to statistics from TUV Germany, engines that maintain the outlet pressure of the oil pump within an error range of ±0.1Bar have an average service life extended to 380,000 kilometers, an increase of 45% compared to the poorly managed control group.