Automotive Service Excellence (ASE) A8 Engine PerformanceFuel, Air Induction, and Exhaust Systems Diagnosis and RepairHard
A vehicle owner reports that their vehicle occasionally experiences hesitation or a 'stumble' during light acceleration, particularly when the engine is cold. There are no diagnostic trouble codes (DTCs) stored. Which of the following is a common cause for this type of issue related to the fuel system?
- AA restricted fuel injector.
- BA weak fuel pump relay.
- CA leaking fuel pressure regulator diaphragm.
- DA completely clogged fuel filter.
Show answer & explanationAnswer & explanation
Correct answer: A. A restricted fuel injector.
A partially restricted fuel injector can cause a lean condition in one or more cylinders, leading to hesitation or a stumble, especially during light acceleration when the ECM is trying to precisely meter fuel. This effect can be more pronounced when cold as the engine requires a richer mixture. Without a complete blockage, a DTC may not be immediately set.
Why the other options are wrong
- B. A weak fuel pump relay would likely cause intermittent or no fuel pump operation, leading to cranking but no start, or complete stalling, not just a stumble during light acceleration.
- C. A leaking fuel pressure regulator diaphragm would typically cause a rich condition (if fuel enters vacuum line) or a lean condition (if pressure is too low), which would likely be more consistent and set DTCs.
- D. A completely clogged fuel filter would cause significant performance issues across the RPM range and likely set lean codes, not just intermittent hesitation when cold.
Restricted Fuel Injector Symptoms
A partially restricted fuel injector can cause engine hesitation, stumble, or misfires, especially under light acceleration or when cold, due to an insufficient fuel supply to one cylinder.
- Causes a lean condition in one cylinder.
- Symptoms can be intermittent or subtle.
- May not immediately set DTCs.
- Can be diagnosed with injector balance test or fuel trim analysis per cylinder.
Memory trick: Hesitation's cause, fuel's pause.