Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisMedium
A technician is diagnosing an engine with a complaint of a consistent, rhythmic 'knocking' sound that is most noticeable when the engine is cold or under light load, and tends to decrease as the engine warms up. The noise sounds somewhat like a piston hitting the cylinder wall. Which of the following is the most likely cause?
- AExcessive valve lash.
- BPiston slap.
- CWorn main bearings.
- DConnecting rod bearing knock.
Show answer & explanationAnswer & explanation
Correct answer: B. Piston slap.
Piston slap is characterized by a knocking sound, often described as a metallic slap, that is more prominent when the engine is cold due to increased piston-to-cylinder wall clearance. As the engine warms up and components expand, the noise typically diminishes.
Why the other options are wrong
- A. Excessive valve lash causes a tapping or clicking noise from the valvetrain, not a piston-like knocking.
- C. Worn main bearings typically produce a deep, dull thud or rumble, not a light knocking that diminishes with warmth.
- D. Connecting rod bearing knock is usually a sharper, metallic knock that is often louder under load and during acceleration/deceleration, and typically doesn't diminish significantly with engine warmth.
Piston Slap
A knocking sound caused by excessive clearance between the piston and the cylinder wall, allowing the piston to 'slap' against the cylinder during its travel.
- Often more noticeable when the engine is cold.
- Tends to decrease or disappear as the engine warms up and components expand.
- Can indicate wear in the piston skirt or cylinder bore.
Memory trick: Listen to the engine, it tells its story.