Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisHard
A technician is diagnosing an engine that exhibits a consistent, rhythmic 'knocking' sound that is loudest at idle and becomes less noticeable as RPM increases. The sound is present whether the engine is hot or cold. What is the MOST likely source of this noise?
- AValve lifter noise.
- BPiston pin knock.
- CMain bearing knock.
- DConnecting rod bearing knock.
Show answer & explanationAnswer & explanation
Correct answer: B. Piston pin knock.
Piston pin knock is characterized by a sharp, metallic rapping sound, often loudest at idle and when the engine is cold, and tends to diminish or disappear under load or as RPM increases due to the changing forces on the piston pin.
Why the other options are wrong
- A. Valve lifter noise is usually a lighter, tapping or ticking sound, often more pronounced when cold or low on oil, and typically does not disappear with increasing RPM like a piston pin knock.
- C. Main bearing knock is a dull, heavy thud that is usually more pronounced under load and increases with RPM, not less noticeable.
- D. Connecting rod bearing knock is a distinct, sharp, metallic double-knock sound that typically increases in intensity and frequency with RPM and is often more noticeable under load.
Piston Pin Knock
A metallic rapping sound caused by excessive clearance between the piston pin and its bore in the piston or connecting rod.
- Often loudest at idle or light load.
- May diminish or disappear as RPM increases or under heavier loads.
- Indicates significant wear and requires engine disassembly for repair.
Memory trick: Knock, Knock! Who's there? Piston Pin, quiet when you accelerate there!