A malfunction of the Fuel Pump will really cause the car to stall under conditions of climbing a hill, especially under the condition of low speed and heavy load. The power output of the engine will be directly affected by the absence of fuel supply pressure. For example, the rated flow rate of the fuel pump for a specific model is 150L/h. But as the impeller wear decreases the volumetric efficiency by 20%, the actual flow rate becomes only 120L/h, which is not enough to meet the instantaneous fuel demand of 135L/h required by the engine at a slope of 15%. The ECU causes a protective shutdown because the air-fuel ratio is over the limit (λ > 1.2). 87,000 units of a North American automaker in 2020 were recalled because their fuel pump impellers have cracked. As per statistics, 23% of complaint cases were on the basis of stalling when ascending. Data stated that the chance of defective models stalling at roads having more than a 10% slope is 4.3 times higher compared to flat roads.
A fuel filter which gets clogged easily will add an extra load on the fuel pump. Test results show that under the condition that the differential pressure of the filter element is greater than 70kPa (normal value < 30kPa), the motor current of the fuel pump rises from 5A to 7.8A, and the winding temperature rises to 120℃ (40℃ higher than the normal operation point), which causes a thermal attenuation effect, leading to a reduction in the output pressure instantly from 3.0bar to 1.8bar. For instance:
Figures from one of the taxi fleets within a specific mountainous area reveal that vehicles with mileage over 100,000 kilometers saw a 37% increase in engine stalling failure while moving uphill as a result of a 50% lengthening of the filter replacement period. From the standard 20,000 kilometers, it’s risen to 30,000 kilometers. Among them, 64% of the vehicles have power attenuation over 15% in the fuel pump.
Defects in the circuit system are also crucial. The 12V voltage powers the fuel pump. When the wiring harness resistance is increased to 1.5Ω (design ≤0.3Ω) by corrosion, the real motor power will be reduced from 80W to 55W, and the flow rate will decrease by 28%. A study by one research institute of 200 engine stall incidents on steep inclines determined that 41% were due to voltage fluctuation. If the altitude rose above 2,000 meters, due to the thin air, 30% less efficiency in heat dissipation of fuel pumps occurred, and the rate of temperature rise of the motor increased by 1.5 times, with the compound effect of further cutting fuel supply stability. Additionally, if the fuel pump control module PWM signal frequency deviation exceeds 10%, the rate of flow regulation error is 25%, and this is extremely likely to cause the engine to stall on steady curves with a slope of 12%.
The fuel pump design parameters as well as their terrain adaptability are significant too. For example, the redundancy flow rate of a specific tough off-road vehicle’s fuel pump is designed to be 180L/h (20% higher than that of urban SUVs). However, in after-market versions, when the maximum pressure of the fuel pump is decreased from 4.0bar to 3.2bar (because of non-standard impellers’ matching fault), the fuel pressure variation range during climbing will rise to ±0.8bar. This made the ECU overestimate the oxygen sensor as faulty and abruptly cut off the oil supply. Industry tests show that when the fuel pump outlet pressure is below 2.5bar, the engine torque output decreases by 18%, and the stalling likelihood on the sections with a slope of 20% increases to 32%.
The relationship between oil quality and the lifespan of fuel pumps cannot be ignored either. High-gum gasoline (gum level > 7mg/100mL) will encourage coking of the fuel pump intake valve, expand the valve seat sealing clearance to 0.08mm from 0.02mm, enhance the internal leakage rate by 15%, and cause a prompt decrease in fuel supply pressure by 40% under conditions of sudden acceleration on uphill grades. For instance, in a Southeast Asian country, due to the use of M15 methanol gasoline, the rate of fuel pump failures is 26% greater than that for cars that use traditional gasoline. Among them, engine stalling on inclining roads accounts for 39% of all failures. The major reason is that the hygroscopicity of methanol leads to internal rusting of the pump body (thickness of rust layer > 0.15mm), and the impeller-casing difference is 50μm higher than the normal standard. Through multi-dimensional data cross-analysis, it is found that the coupling relationship between fuel pump performance degradation, terrain, load and maintenance cycle is one of the primary causes of engine stalling on uphill slopes.