Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisHard
A vehicle exhibits a severe, rhythmic 'thump' or 'clunk' noise that is directly proportional to engine RPM, present at all times, and sounds like it's coming from deep within the engine block. The noise does not change significantly with engine load or temperature. What is the MOST likely cause?
- AA loose accessory drive pulley.
- BAn exhaust manifold leak.
- CA worn connecting rod bearing.
- DA failed hydraulic lifter.
Show answer & explanationAnswer & explanation
Correct answer: C. A worn connecting rod bearing.
A deep, rhythmic 'thump' or 'clunk' from within the engine block, directly proportional to RPM and not changing with load, is characteristic of a worn connecting rod bearing. This is a severe internal engine mechanical failure, as the bearing clearance allows the connecting rod to 'knock' against the crankshaft.
Why the other options are wrong
- A. A loose accessory drive pulley might cause a squeal, rattle, or wobble, but not a deep, rhythmic internal engine 'thump'.
- B. An exhaust manifold leak would cause a 'ticking' or 'puffing' sound, typically from the exhaust side, and would often change with load.
- D. A failed hydraulic lifter typically causes a 'tapping' or 'ticking' noise, usually from the top of the engine, not a deep 'thump'.
Connecting Rod Bearing Knock
A severe engine noise caused by excessive clearance between the connecting rod and crankshaft journal due to bearing wear.
- Deep, heavy, rhythmic 'thump' or 'clunk' noise.
- Directly proportional to engine RPM.
- Often heard from deep within the engine block.
- Indicates serious internal engine damage requiring immediate attention.
Memory trick: Deep 'thump' with RPM's call, rod bearing's about to fall!