Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisHard
A technician is diagnosing an engine that exhibits a severe, rhythmic 'thump' or 'clunk' noise that is directly proportional to engine RPM and becomes louder under load. The noise is present at all engine speeds but is most concerning when accelerating. A cylinder cut-out test reveals that disabling cylinder #3 significantly reduces the noise. What is the most likely cause?
- AWorn connecting rod bearing on cylinder #3
- BWorn main bearing on cylinder #3
- CLoose piston pin on cylinder #3
- DCollapsed hydraulic lifter on cylinder #3
Show answer & explanationAnswer & explanation
Correct answer: A. Worn connecting rod bearing on cylinder #3
A sharp, metallic 'thump' or 'clunk' that is directly proportional to engine RPM, louder under load, and significantly reduced by cutting out a specific cylinder points to a worn connecting rod bearing in that cylinder. The force of combustion on the piston and connecting rod causes the worn bearing to knock.
Why the other options are wrong
- B. Worn main bearings typically produce a dull, heavy rumble, and while it would be louder under load, a cylinder cut-out test would not typically isolate it to a single cylinder as directly as a rod knock.
- C. A loose piston pin usually causes a lighter, double-knock that is often loudest at idle and may diminish with increasing RPM, and while a cut-out test would reduce it, the 'severe thump' and 'louder under load' better fit a rod bearing.
- D. A collapsed hydraulic lifter would cause a ticking or clattering noise, not a severe 'thump' or 'clunk'.
Connecting Rod Bearing Knock
A sharp, metallic 'knock' or 'clunk' sound that is usually directly proportional to engine RPM, becomes louder under load, and can often be isolated to a specific cylinder via a cut-out test.
- Sharp, metallic, rhythmic knock/clunk.
- Directly proportional to engine RPM.
- Louder under load.
- Often reduced by disabling the affected cylinder.
Memory trick: RPM-tied clunk, cylinder cut, rod bearing's sunk!