Unstable fuel pressure can lead to deceleration and backfire. When the output pressure of the fuel pump fluctuates by more than ±0.3Bar (the normal range is 2.8-4.2Bar), the air-fuel ratio will instantly deviate from the theoretical value by more than 14.7:1 to 18:1 (JASO M342-2023 standard). Ford’s 2025 technical report shows that due to the pressure drop caused by plunger wear (up to 1.6Bar), 22% of the unburned mixture enters the exhaust pipe at the moment the throttle closes. After coming into contact with the 800 ° C high-temperature pipe wall, the probability of deflagring is as high as 73%. In the actual test case, when the Nissan HR16DE engine’s rotational speed suddenly dropped (from 5000 to 1500rpm), the low-pressure fuel triggered the ECU’s thickening compensation to fail, with a backfire frequency of 4.8 times per minute.
The main cause is the fuel siphoning caused by the failure of the check valve. The leakage rate of the check valve of the inferior fuel pump is > 30ml/min (the standard value is < 5ml/min), and the oil rail pressure leaks to 0Bar within 5 seconds after stopping (the standard requirement is to maintain > 1.5Bar/30min). General Motors’ 2024 recall incident confirmed that the rubber aging of the check valve (Shore hardness < 65HA) caused a delay of 0.8 seconds in the establishment of oil pressure during hot start and a 19% deviation in fuel injection volume during the deceleration stage. At this point, the vacuum degree of the intake manifold rose to 65kPa (the average during normal driving was 35kPa), and the unatomized fuel was sucked into the exhaust manifold, triggering the peak deflagrant temperature of 1062℃ at the oxygen sensor (data from the infrared thermal imager).
Inaccurate flow control intensifies the risk of backfire. When the fuel pump wear causes the maximum flow rate to drop to 90L/h (the original factory design value ≥130L/h), the ECU closed-loop control overshoot increases by 40%. The bench test of the Mitsubishi 4G63 engine shows that when the throttle suddenly changes, the fuel response lags by 280ms (standard < 120ms), and the fluctuation amplitude of the air-fuel ratio increases from ±0.8 to ±2.5. The after-sales maintenance statistics of the BMW N20 engine show that 23% of the deceleration backfire faults are caused by the impeller clearance exceeding the tolerance (> 0.15mm), the oil pump efficiency drops to 65% (for new pumps > 85%), and the failure rate of the deceleration fuel cut-off strategy execution rises to 31%.
The vacuum condition linkage mechanism cannot be ignored. The vacuum value of the intake manifold can reach -80 kpa during sudden deceleration. If the fuel pump pressure relief valve gets stuck (opening degree < 85%), the fuel rail pressure soaps to 6.5Bar (exceeding the limit by 61%), and the overinjection fuel volume reaches 3.7ml per minute. According to the study of SAE Paper 2024-01-1615, at this time, the fuel injector shutdown delay Angle expanded to 1.8°CA (normal 0.3°CA), and the probability that the residual fuel evaporated and was sucked into the exhaust pipe under negative pressure increased by 67%. The Toyota Corolla maintenance case library shows that after replacing the fuel pump that complies with the JIS D1617 specification, the complaint rate of backfire decreased from 1.2% per month to 0.08%.