Automotive Service Excellence (ASE) A8 Engine PerformanceComputerized Engine Controls Diagnosis and RepairHard
A technician is diagnosing a vehicle that started exhibiting a 'hard start' condition, requiring extended cranking, especially after sitting overnight. Once started, the engine runs normally, and there are no DTCs. The technician performs a fuel pressure test and observes that the fuel pressure drops rapidly from 50 PSI to 0 PSI within 5 minutes after the engine is shut off. What is the MOST likely cause of this symptom?
- AA faulty fuel pump check valve.
- BA faulty fuel pressure regulator.
- CA clogged fuel filter.
- DA restricted fuel return line.
Show answer & explanationAnswer & explanation
Correct answer: A. A faulty fuel pump check valve.
A rapid drop in fuel pressure after engine shutoff indicates a loss of fuel system integrity, specifically the inability to hold residual pressure. The fuel pump's check valve is responsible for maintaining this residual pressure in the fuel lines. A faulty check valve allows fuel to drain back to the tank, leading to a hard start as the pump has to re-prime the system.
Why the other options are wrong
- B. A faulty fuel pressure regulator could cause either excessively high or low pressure during operation, or bleed off pressure if stuck open, but the check valve is the more common component for rapid pressure loss after shutdown.
- C. A clogged fuel filter would primarily cause low fuel pressure during engine operation, not a rapid drop after shutdown.
- D. A restricted fuel return line would cause excessively high fuel pressure, not a rapid drop after engine shutdown.
Fuel System Residual Pressure
Residual fuel pressure is the pressure maintained in the fuel system after the engine is shut off, primarily by the fuel pump's check valve. It prevents fuel from draining back to the tank and ensures quick starts.
- Maintained by the fuel pump check valve.
- Prevents vapor lock and aids quick starts.
- Rapid drop indicates a leak or faulty check valve.
Memory trick: Pressure drops fast? Check the valve, it's letting fuel pass!