Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisMedium
A technician is diagnosing an engine with a complaint of a consistent, loud 'rumbling' or 'growling' noise that is most prominent when the engine is under load, such as driving uphill or accelerating. The noise is less noticeable at idle or during light cruising. What is the most likely cause?
- AWorn connecting rod bearings
- BWorn main bearings
- CFailed catalytic converter
- DLoose accessory drive belt
Show answer & explanationAnswer & explanation
Correct answer: B. Worn main bearings
Worn main bearings typically produce a heavy, dull 'thumping' or 'rumbling' sound that is more pronounced under engine load because the crankshaft is pressing harder against the worn bearings.
Why the other options are wrong
- A. Worn connecting rod bearings usually cause a sharp, metallic 'knock' that is directly proportional to engine RPM and often more noticeable at certain loads or decel.
- C. A failed catalytic converter (clogged or broken substrate) might cause power loss and exhaust restriction noise, but not an internal engine 'rumbling' related to bearings.
- D. A loose accessory drive belt would typically cause a squealing or chirping noise, especially on startup or during heavy accessory load, not a deep rumble under engine load.
Main Bearing Knock/Rumble
A deep, dull, heavy knocking or rumbling sound from the lower part of the engine, typically caused by excessive clearance in the main bearings.
- Dull, heavy thumping or rumbling.
- Most noticeable under heavy engine load (acceleration, uphill).
- May be less noticeable at idle or light load.
Memory trick: Load makes it rumble? Main bearings are grumble.