ASE A1 Engine RepairGeneral Engine DiagnosisMedium
A customer complains of a persistent, loud 'thumping' or 'pounding' noise coming from the lower part of the engine, which increases in frequency with engine RPM and is especially noticeable under load. What is the MOST probable cause?
- ALoose accessory drive belt
- BStuck open EGR valve
- CWorn crankshaft main bearings
- DFailed water pump bearing
Show answer & explanationAnswer & explanation
Correct answer: C. Worn crankshaft main bearings
A deep 'thumping' or 'pounding' noise from the lower engine that increases with RPM and is pronounced under load is a classic symptom of worn crankshaft main bearings. These bearings support the crankshaft and their wear creates excessive clearance.
Why the other options are wrong
- A. A loose accessory drive belt typically causes a squealing noise, especially on startup or when accessories are loaded.
- B. A stuck open EGR valve causes rough idle, hesitation, and possibly misfires, but not a mechanical thumping noise.
- D. A failed water pump bearing usually produces a whining or grinding noise from the front of the engine, not a deep thumping.
Worn Engine Bearings Diagnosis
Worn engine bearings (main or connecting rod) cause distinct internal engine noises that vary with engine speed and load.
- Main bearings: Deep 'thumping' or 'pounding' from lower engine.
- Connecting rod bearings: Sharper 'knocking' or 'tapping' from lower engine.
- Noise increases with RPM and is often worse under load.
Memory trick: Lower thump, main bearing's the crumb.