ASE A1 Engine RepairEngine Block AssemblyEasy
A client complains of an engine making a 'slapping' noise, especially when cold, which lessens as the engine warms up. A technician suspects a piston-related issue. Which of the following is the MOST likely cause?
- AStuck open hydraulic lifter.
- BExcessive piston-to-cylinder wall clearance.
- CExcessive crankshaft end play.
- DWorn main bearings.
Show answer & explanationAnswer & explanation
Correct answer: B. Excessive piston-to-cylinder wall clearance.
A 'piston slap' noise, which is typically loudest when cold and diminishes as the engine warms up, is characteristic of excessive piston-to-cylinder wall clearance. As the engine heats up, the piston expands, reducing the clearance and thus the noise.
Why the other options are wrong
- A. A stuck open hydraulic lifter would cause a ticking or tapping noise, not a slapping sound.
- C. Excessive crankshaft end play would typically cause a dull thud or knock, not a slap, and wouldn't necessarily lessen with temperature.
- D. Worn main bearings would cause a deep, heavy knock or thud that is usually constant or worsens with engine speed/load.
Piston Slap
Piston slap is a metallic knocking sound caused by excessive clearance between the piston skirt and the cylinder wall, allowing the piston to rock and hit the cylinder wall.
- Often more noticeable when the engine is cold due to less piston expansion.
- Can be caused by wear, incorrect machining, or incorrect piston size.
- Long-term, it can lead to increased oil consumption and cylinder wear.
Memory trick: Piston slap sounds like a 'cold hand' hitting the wall.