Internal leakage of the Fuel Pump is the main cause of difficult hot start of the vehicle. Statistics from a certain auto repair shop show that 82% of the cases of delayed start are accompanied by prolonged establishment time of oil pressure. At high temperatures, the wear clearance of the plunger pump expands to 0.15mm (0.02mm for the new pump). The measured data of the Porsche 911 shows that when the ambient temperature rises from 25℃ to 85℃, the hot start fuel pressure drops from 3.5bar to 1.2bar, and the fuel rail filling time increases from 2.3 seconds to 7.1 seconds.
The problem of insufficient flow during gasoline vaporization intensifies. At 60℃, the saturated vapor pressure of fuel can reach 45kPa (only 15kPa at 20℃). Tests of the Volkswagen TSI engine have shown that when the outlet temperature of the fuel pump exceeds 75℃, the cavitation flow loss reaches 38%, and the actual fuel supply at 1500rpm is only 12L/min (the required 18L/min), causing the air-fuel ratio to deteriorate from 14.7:1 to 17:1.
Thermal deformation alters the sealing characteristics of the valve body. Under high-temperature conditions, the expansion coefficient of the brass check valve is 1.9×10⁻⁵/℃, with a difference rate of 21% compared to the cast aluminum pump body (2.3×10⁻⁵/℃). The modern Sonata case shows that at 90℃, the sealed contact surface is reduced by 63%, the system’s pressure-maintaining capacity decays from 3bar in 30 minutes after shutdown to 0.8bar, and it takes more than 1.2 seconds to rebuild the start-up oil pressure.
The carbon powder contamination of the brush causes a sharp increase in conductivity at high temperatures. After 130,000 kilometers, carbon deposits on the commutator of the oil pump motor of the Mercedes-Benz M274 engine cause the winding resistance to shift by +18%. The leakage current in the hot start state increased from 3mA to 85mA, the effective power loss in the start-up stage was 32% (from 200W to 136W), and the speed response delay was more than 0.4 seconds.
The failure of the cooling system triggered a vicious cycle. A certain car owner installed a heat insulation cover, causing the ambient temperature of the oil pump to rise by 72℃ (the original design peak was 62℃). The infrared thermal image shows that the surface temperature of the pump body is 126℃, the internal fuel temperature is 93℃, and the slope of the flow attenuation curve increases by 45%. After removing the heat shield, the startup time returned to the normal value in 1.8 seconds (originally it required 4.5 seconds).
The design defect of the oil circuit leads to accelerated reflux, and the General ECOTEC series has the problem of improper position of the check valve. Thermal expansion reduces the sealing force of the valve ball by 40%. Within 15 minutes after the shutdown, the oil rail pressure drops from 300bar to 120bar (normally it should be > 200bar), and an additional 3.2 seconds are required to rebuild the oil pressure. This defect led to the recall of 140,000 Cruzes in 2018.
Diagnostic tests can quickly identify the source of faults: When the engine is turned off in the hot state and the oil pressure drop within 120 seconds is greater than 15bar (standard < 8bar), it can be determined that the Fuel Pump has failed. The BMW ISTA testing protocol requires that the starting current waveform be measured simultaneously: normally, it should be a peak of 8A lasting for 0.6 seconds. If a collapse wave less than 7.2A appears, it confirms that the motor has deteriorated.