Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisMedium
A technician is diagnosing an engine that occasionally stalls at idle and has a consistent, rhythmic 'thumping' noise that is most pronounced at idle and under light load. The noise seems to originate from the lower part of the engine and its frequency is roughly half the engine's RPM. What is the most likely cause?
- AA faulty fuel injector.
- BWorn main bearings.
- CExcessive valve lash.
- DWorn connecting rod bearings.
Show answer & explanationAnswer & explanation
Correct answer: D. Worn connecting rod bearings.
A rhythmic 'thumping' noise from the lower engine, especially at idle and light load, with a frequency roughly half engine RPM, is characteristic of worn connecting rod bearings. These bearings connect the piston to the crankshaft, and wear causes play and noise.
Why the other options are wrong
- A. A faulty fuel injector would cause a misfire and rough running, but not a distinct mechanical thumping noise from the lower engine.
- B. Worn main bearings typically produce a deeper, duller 'rumble' or 'thud', not a distinct 'thumping' at half engine RPM.
- C. Excessive valve lash would cause a ticking or clicking noise from the top of the engine, not a thumping from the lower engine.
Connecting Rod Bearing Knock
A rhythmic, metallic 'thump' or 'knock' noise from the lower engine, caused by excessive clearance in the connecting rod bearings.
- Often more noticeable at idle, light load, or during acceleration/deceleration.
- Frequency is typically half engine RPM (once per crankshaft revolution).
- Can lead to severe engine damage if not addressed.
Memory trick: Rods thump, mains rumble, don't stumble.