ASE A1 Engine RepairGeneral Engine DiagnosisHard
A technician is diagnosing an engine with a persistent 'thumping' or 'pounding' noise coming from the lower part of the engine, which increases in intensity with RPM. The noise is duller and deeper than a 'ping' or 'tick'. What is the MOST likely cause?
- ALoose accessory pulley
- BWorn camshaft lobes
- CWorn main or connecting rod bearings
- DExhaust manifold leak
Show answer & explanationAnswer & explanation
Correct answer: C. Worn main or connecting rod bearings
A deep 'thumping' or 'pounding' noise from the lower engine, increasing with RPM, is characteristic of worn main or connecting rod bearings. These components, when worn, create excessive clearance, leading to a dull, heavy impact sound.
Why the other options are wrong
- A. A loose accessory pulley usually causes a squealing, chirping, or sometimes a rattling noise, not a deep pounding sound from within the engine.
- B. Worn camshaft lobes typically cause a ticking or tapping noise from the valvetrain, not a deep thumping from the lower engine.
- D. An exhaust manifold leak typically produces a ticking or hissing sound, often more pronounced when cold, not a deep thumping.
Worn Engine Bearings
Worn engine bearings (main or connecting rod) have excessive clearance, causing a metallic contact between moving parts and producing a distinct low-frequency knocking sound.
- Causes a deep 'thump' or 'pound' noise from the lower engine.
- Noise typically increases in frequency and intensity with RPM.
- Can lead to catastrophic engine failure if not addressed.
Memory trick: Lower engine thump, bearings are worn, time to jump.