ASE G1 Auto Maintenance and Light RepairEngine SystemsMedium
A customer reports that their vehicle's engine occasionally 'pings' or 'knocks' under acceleration, and fuel economy has worsened. The engine also seems to lack its usual power. There are no diagnostic trouble codes stored. Which of the following is most likely causing these symptoms?
- ALow-octane fuel being used
- BIncorrect spark plug gap
- CA failing camshaft position sensor
- DA clogged PCV valve
Show answer & explanationAnswer & explanation
Correct answer: A. Low-octane fuel being used
Engine pinging/knocking under acceleration, reduced power, and poor fuel economy, especially without DTCs, are classic symptoms of using fuel with an octane rating lower than what the engine requires. Lower octane fuel ignites prematurely, causing pre-ignition.
Why the other options are wrong
- B. Incorrect spark plug gap can cause misfires or rough running, but typically not consistent pinging across the board without DTCs, and often not severe fuel economy loss.
- C. A failing camshaft position sensor would likely cause hard starting, stalling, or misfires, and almost certainly trigger a DTC.
- D. A clogged PCV valve typically causes oil leaks, rough idle, or increased oil consumption, not pinging and power loss without DTCs.
Low Octane Fuel Effects
Using fuel with an octane rating lower than recommended can cause engine 'pinging' or 'knocking' (pre-ignition), reduced power, and decreased fuel efficiency as the engine's control system attempts to compensate.
- Octane resists pre-ignition.
- Low octane causes 'pinging' and 'knocking'.
- Engine computer retards timing to compensate, reducing power/efficiency.
Memory trick: Pinging means the fuel is singing the wrong tune.