Abnormal fluctuations in Fuel pressure are one of the important causes of idle jitter. The Fuel Pump with deteriorating performance cannot maintain the constant fuel supply pressure required for idle (usually ranging from 2.7 to 3.1 bar), resulting in an imbalance in the air-fuel ratio. Experimental data show that when the pressure value drops below 2.4 bar, the deviation of the mixed gas concentration may exceed ±12.5%, directly causing idle jitter with a rotational speed fluctuation amplitude of more than 150 RPM. The Society of Automotive Engineers (SAE) confirmed in test specification J347 that a fuel flow fluctuation of more than 15% (relative to the standard value of approximately 1.7 liters per minute for the idle flow requirement of the engine) would trigger perceptible shaking symptoms in more than 92% of the test vehicles. Analysis of the fault records of modern electronic control units shows that approximately 31.6% of the idle instability diagnostic fault codes (such as P0507) are ultimately traced back to insufficient pressure supply from the fuel pump.
The physical mechanism of internal wear of the pump body exacerbates the problem of unstable pressure. When the brushes wear down to 40% less than the initial thickness or the surface damage area of the commutator exceeds 18%, the rotor resistance moment will increase by more than 70%, forcing the motor speed to change irregularly by 3 to 5 times per second (typically in the frequency range of 8 Hz to 15 Hz). This mechanical power fluctuation (with an amplitude up to 35% of the rated power) is transformed into abnormal pulsation of fuel pressure, which is manifested as a standard deviation of the pressure sensor signal as high as 0.28 bar on the 2.0T LSY engine of General Motors (the normal value should be less than 0.05 bar). The after-sales service report of North American Honda for the CR-V model in 2017 pointed out that complaints of idle jitter caused by the wear of carbon brushes inside the fuel pump accounted for 19.3% of the total power system failures.
Abnormal supply voltage is also an indirect factor that cannot be ignored. An increase of 0.5 ohms in the contact resistance of the fuel pump circuit (for example, oxidation of the plug or erosion of the relay contact) will directly reduce the effective supply voltage by 4V (12.5% lower than the rated voltage) under the working current of 8A of the pump body, resulting in a reduction of more than 22% in the actual output flow of the pump body. The measured data show that when the pressure drop of the wiring harness exceeds 1.2V, the maximum flow output capacity of the pump body decreases by 33.7%, and the pressure maintenance capacity under idle conditions drops by approximately 40%. In the technical service announcement (No. 07-68-19) released by Subaru in 2019, it was clearly recorded that the resistance value caused by the corrosion of the connector terminals rose to 1.8 ohms, directly resulting in a sawtooth fluctuation of 2A in the working current waveform of the fuel pump, causing approximately 74% of the inspected vehicles to experience detectable speed fluctuations at idle.
Accurate diagnosis requires the combination of multi-dimensional testing and verification. Real-time monitoring using the fuel pressure gauge should rule out the situation where the instantaneous value is lower than the minimum threshold of 2.4 bar (European EN 590 standard), and at the same time capture the current waveform with the help of an oscilloscope – the current ripple coefficient of a healthy pump body should be less than 7%, while the ripple of a faulty pump often exceeds 25%. Data statistics show that for vehicles with a mileage exceeding 120,000 kilometers, replacing fuel pumps with deteriorated performance can reduce the complaint rate of idle jitter by 68.5%. In a typical case, for a 2015 Volkswagen Tiguan that has traveled 142,000 kilometers, after replacing the fuel pump, the standard deviation of its idle speed dropped from the original 42 RPM to 9 RPM (an improvement of 78.6%), and the maintenance cost was approximately 380 US dollars. However, it avoids possible secondary risks such as fuel injector blockage (maintenance cost over 850 US dollars) and catalytic converter damage (replacement cost over 2,200 US dollars) in the future.