Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisMedium
A technician is diagnosing an engine that exhibits a consistent, rhythmic 'knocking' sound that is loudest at idle and disappears or significantly diminishes as engine RPM increases. What is the most likely cause of this noise?
- ACollapsed hydraulic lifter
- BLoose piston pin
- CExhaust manifold leak
- DWorn main bearings
Show answer & explanationAnswer & explanation
Correct answer: B. Loose piston pin
A loose piston pin typically causes a double-knock or a light, sharp knock that is most noticeable at idle and often becomes less apparent or disappears as engine RPM increases due to increased cylinder pressure loading the pin.
Why the other options are wrong
- A. A collapsed hydraulic lifter would cause a ticking or clattering noise, not a knocking sound.
- C. An exhaust manifold leak would produce a 'hissing' or 'ticking' sound, especially noticeable under acceleration, not a rhythmic knock.
- D. Worn main bearings usually produce a heavy, dull knock that tends to increase with engine speed and load.
Piston Pin Knock
A metallic knocking sound caused by excessive clearance between the piston pin and its bore in the piston or connecting rod.
- Often a double-knock or light, sharp knock.
- Most noticeable at idle.
- May disappear or diminish as engine RPM increases.
Memory trick: Piston Pin: Idle's light knock, then it's gone.