Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisMedium
A technician is diagnosing an engine that has a rhythmic 'thump' or 'clunk' noise that is directly proportional to engine RPM and becomes louder under load. The noise is present at all engine temperatures. What is the MOST likely cause?
- AValve lifter noise.
- BPiston slap.
- CConnecting rod bearing knock.
- DMain bearing knock.
Show answer & explanationAnswer & explanation
Correct answer: C. Connecting rod bearing knock.
A rhythmic 'thump' or 'clunk' that is directly proportional to engine RPM and becomes louder under load is characteristic of a connecting rod bearing knock, indicating excessive clearance in the rod bearings.
Why the other options are wrong
- A. Valve lifter noise is typically a lighter 'tick' or 'tap' and is not usually described as a 'thump' or 'clunk' that gets louder under load.
- B. Piston slap is a metallic 'slapping' sound, often loudest when cold and at idle, and tends to diminish as the engine warms up or under load, which is opposite to the described behavior.
- D. Main bearing knock is a dull, heavy 'thud' or 'rumble' that is usually more pronounced under load and increases with RPM, but is generally heavier and less sharp than a rod knock.
Connecting Rod Bearing Knock
A distinct, rhythmic metallic knocking sound caused by excessive clearance in the connecting rod bearings.
- Directly proportional to engine RPM.
- Becomes louder and more pronounced under engine load.
- Often described as a 'double-knock' or 'clunk'.
- Requires engine disassembly for repair.
Memory trick: Rod's Rhythmic Clunk, Load it Up, it's a Thump!