Automotive Service Excellence (ASE) A8 Engine PerformanceFuel, Air Induction, and Exhaust Systems Diagnosis and RepairMedium
A vehicle owner reports that their vehicle occasionally experiences a noticeable loss of power and hesitation during acceleration, particularly when the fuel tank is less than a quarter full. The issue is intermittent and does not consistently trigger any diagnostic trouble codes (DTCs). When the fuel tank is full, the problem is rarely observed. What is the MOST likely cause of this complaint?
- AA faulty fuel pressure regulator.
- BA clogged fuel injector.
- CA failing in-tank fuel pump.
- DA restricted fuel return line.
Show answer & explanationAnswer & explanation
Correct answer: C. A failing in-tank fuel pump.
A failing in-tank fuel pump, especially one that is overheating or has a weak motor, often struggles more when the fuel level is low because the pump relies on the surrounding fuel for cooling. When the tank is full, it's better cooled and submerged, masking the issue. This leads to intermittent fuel starvation and power loss when the fuel level drops.
Why the other options are wrong
- A. A faulty fuel pressure regulator would typically cause consistent high or low fuel pressure, regardless of fuel level, and likely more consistent symptoms or DTCs.
- B. A clogged fuel injector would typically cause a misfire on a specific cylinder and would not be dependent on the fuel tank level.
- D. A restricted fuel return line would cause excessively high fuel pressure, potentially leading to rich conditions or hard starting, not issues related to low fuel levels.
In-Tank Fuel Pump Failure (Low Fuel)
A common symptom of a failing in-tank fuel pump where performance issues (hesitation, power loss) occur when the fuel level is low, due to insufficient cooling of the pump motor.
- Fuel pumps are cooled by the fuel in the tank.
- Low fuel levels can lead to pump overheating.
- Symptoms are often intermittent and worsen with low fuel.
Memory trick: Low fuel, hot pump, power slump!