Automotive Service Excellence (ASE) A8 Engine PerformanceFuel, Air Induction, and Exhaust Systems Diagnosis and RepairHard
A technician is performing a fuel pressure test on a gasoline direct injection (GDI) system. The specified low-pressure fuel pump output is 60-70 PSI, and the high-pressure fuel pump output is 500-2000 PSI, depending on demand. If the technician measures the low-pressure fuel pump at 45 PSI and the high-pressure fuel pump at 300 PSI at idle, what is the MOST likely cause of the low high-pressure reading?
- AA faulty high-pressure fuel pump.
- BA restricted fuel return line.
- CInsufficient fuel supply from the low-pressure pump.
- DA clogged fuel filter in the high-pressure side.
Show answer & explanationAnswer & explanation
Correct answer: C. Insufficient fuel supply from the low-pressure pump.
The high-pressure GDI pump relies on a sufficient supply of fuel from the low-pressure pump. If the low-pressure pump is only supplying 45 PSI when 60-70 PSI is specified, the high-pressure pump will be starved of adequate fuel, preventing it from reaching its target pressure, even if it is otherwise functional.
Why the other options are wrong
- A. While a faulty high-pressure pump is possible, the low primary pressure reading points to a supply issue first. The high-pressure pump can only boost what it receives.
- B. A restricted fuel return line would typically cause excessively high fuel pressure, not low pressure.
- D. GDI systems typically do not have a separate 'high-pressure side' fuel filter. The main fuel filter is usually in the low-pressure circuit.
GDI Fuel Pressure Interdependence
In a GDI system, the high-pressure fuel pump relies on adequate fuel supply from the low-pressure fuel pump. Insufficient low-pressure supply will directly limit the high-pressure output.
- GDI uses two fuel pumps: low-pressure (tank to HP pump) and high-pressure (HP pump to injectors).
- Low-pressure pump typically 40-80 PSI.
- High-pressure pump typically 500-3000 PSI.
- Diagnose low-pressure side first if both are low.
Memory trick: Low flow in, low pressure out.