Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisHard
A vehicle exhibits a consistent, dull, heavy 'thud' or 'rumble' that is proportional to engine RPM and becomes significantly more pronounced under heavy engine load. The noise is originating from the lower part of the engine block. What is the MOST likely cause?
- AValve lifter noise.
- BPiston pin knock.
- CMain bearing knock.
- DConnecting rod bearing knock.
Show answer & explanationAnswer & explanation
Correct answer: C. Main bearing knock.
A dull, heavy 'thud' or 'rumble' originating from the lower engine block, proportional to RPM, and becoming significantly louder under heavy load, is characteristic of main bearing knock due to excessive clearance.
Why the other options are wrong
- A. Valve lifter noise is a light 'tick' or 'tap' from the cylinder head area, not a heavy 'thud' from the lower block.
- B. Piston pin knock is a sharper, metallic rap, often loudest at idle and diminishing under load, usually not a dull thud from the lower block.
- D. Connecting rod bearing knock is a sharper, often 'double-knock' or 'clunk', and while proportional to RPM and louder under load, it's typically not described as a 'dull, heavy thud' originating from the very bottom of the engine.
Main Bearing Knock
A heavy, dull 'thud' or 'rumble' originating from the lower engine block, caused by excessive clearance in the main crankshaft bearings.
- Directly proportional to engine RPM.
- Significantly louder under heavy engine load.
- Indicates severe wear and requires engine disassembly for repair.
- Often felt as a vibration in the engine as well.
Memory trick: Main's Dull Thud: Load it Heavy, it's a Jud!