The core factor is the attenuation of the Fuel system’s pressure-holding capacity. After the Fuel Pump was replaced, the compression of the sealing ring was less than 0.25mm (the standard is 0.5-0.8mm), causing the rail pressure to drop sharply from 58.5 psi to 22 psi within 5 minutes of the 60℃ hot car being turned off (the normal attenuation should be > 40 psi). At this point, the time required to rebuild the pressure for startup has been extended from 0.8 seconds to 3.5 seconds, and the fuel injection volume is instantly reduced by 38%. The 2023 SAE research report indicates that the leakage probability of inferior installation kits has reached 47%, causing the failure rate of hot starts to soar to 650% of the daily value.
The vapor barrier effect is amplified in high-temperature environments. When the heat insulation oil pipe is not replaced, the 90℃ thermal radiation in the engine compartment causes the fuel vapor pressure to rise to 48 kPa (23 kPa for cold fuel), and the cavitation density at the oil pump inlet is ≥80/cm³. Actual measurement shows that when the oil temperature is 80℃, the flow rate drops by 53%, while qualified insulation tapes can keep the oil pipe temperature below 65℃ and reduce the steam generation by 70%. The engine maintenance data of BMW N55 shows that after optimizing the heat insulation, the compensation accuracy of the hot start fuel injection pulse width has been improved to ±0.1ms (originally ±1.2ms).
The voltage matching defect delays the voltage build-up. The internal resistance of the new oil pump decreases by 0.2Ω and requires compensation current. However, the use of the old wire diameter (18AWG) leads to an increase of 0.8V in the line voltage drop. The current demand at hot start-up is 12A, but the actual power supply is only 10.2A. The speed climbing rate drops from 18,000 rpm/s to 7,500 rpm/s. Audi Q7 actual test: After upgrading the 16AWG cable, the fuel pump pressure building time was reduced from 2.4 seconds to 0.9 seconds, and the ECU fuel correction value returned to the ±5% range.
The fuel injector parameters were not recalibrated, causing venting. The flow rate of the high-pressure oil pump increased by 25%, but the ECU still controlled according to the original pulse width reference. The error of the first cycle injection volume reached +22%. The density difference between hot and cold oil (0.78→0.72 g/cm³) exacerbated the deviation, and the air-fuel ratio during hot start shifted to 17.8:1. The case of Toyota Camry confirmed that after repainting the ECU fuel level map, the standard deviation of the starting speed fluctuation was compressed from ±125 rpm to ±15 rpm.
The air lock phenomenon is more pronounced in modified pumps. For large-flow impellers (such as Walbro 255LPH), when the residual pressure in the oil tank is 0.3 bar, a vortex vacuum degree of -1.2 bar is generated, and the dissolved air release rate exceeds 120%. After the vehicle is warmed up and left to stand, the volume of air bubbles in the oil pipe accounts for 19% (safety limit < 5%), and 3 to 5 ignition attempts are required to disperse the air bubbles. Solution: Install a 0.5L pressure storage tank to maintain a pressure of 50 psi, which will increase the success rate of a single hot start by 98%.
The difference in the coefficient of thermal expansion induces leakage. The expansion rate difference between the aluminum alloy pump body and the plastic oil tank is 0.012%/℃ (23ppm for aluminum alloy vs. 110ppm for plastic), and the sealing gap expands by 0.15mm at 60℃. Mercedes-benz M276 engine test: For vehicles without special sealant, the probability of hot car leakage is 83%, and the fuel vapor concentration exceeds 8000ppm (the national standard limit is 2000ppm). After standardized construction, the leakage rate was reduced to zero, and the HC emissions at startup were decreased by 92%.
The intelligent control system can partially compensate. The integrated pressure maintenance valve (maintaining pressure > 35 psi) reduces the recovery time of hot start oil pressure to 0.3 seconds. The technical notice of Porsche 991.2 confirms that after enabling this function, the hot start time has been reduced from 4.2 seconds to 1.5 seconds, with a success rate of 100%. Simultaneously upgrade the temperature-pressure compensation algorithm, with a cost of approximately ￥1200 but saving 90% of the repair costs.