Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisHard
A client reports that their vehicle's engine exhibits a dull, heavy 'thud' or 'rumble' noise that is most pronounced at idle and under load, and its frequency directly correlates with engine RPM. This noise is typically heard from the lower part of the engine. Which of the following is the most likely cause?
- APiston slap.
- BLoose accessory drive belt.
- CWorn main bearings.
- DStuck hydraulic lifter.
Show answer & explanationAnswer & explanation
Correct answer: C. Worn main bearings.
A dull, heavy 'thud' or 'rumble' from the bottom of the engine, especially pronounced at idle and under load, and directly correlating with RPM, is characteristic of worn main bearings. These bearings support the crankshaft and their wear allows excessive movement.
Why the other options are wrong
- A. Piston slap is a lighter, sharper metallic slap, often more noticeable when cold, and not typically described as a 'dull thud' or 'rumble'.
- B. A loose accessory drive belt typically causes a squealing or chirping noise, not a heavy thud.
- D. A stuck hydraulic lifter usually causes a tapping or clicking noise from the top of the engine.
Main Bearing Knock/Rumble
A deep, dull, heavy thud or rumble noise originating from the lower engine block, caused by excessive clearance in the main bearings supporting the crankshaft.
- Often most noticeable at idle and under load.
- Frequency increases with engine RPM.
- Indicates serious internal engine wear and requires immediate attention.
Memory trick: Knocks and rumbles tell tales of parts.