Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisEasy
A technician observes an engine that has a slight, consistent vibration at idle. Using a stethoscope, the technician isolates the noise to the area of the accessory drive belt tensioner. The vibration does not significantly change with engine RPM, and no specific misfire is detected. What is the most likely cause?
- AAn imbalanced flywheel.
- BA bent crankshaft.
- CA faulty accessory drive belt tensioner.
- DA worn main bearing.
Show answer & explanationAnswer & explanation
Correct answer: C. A faulty accessory drive belt tensioner.
A slight, consistent vibration isolated to the accessory drive belt tensioner, which doesn't significantly change with RPM, strongly suggests a failing tensioner. The tensioner's internal components can wear, causing a vibration or noise that transmits through the engine.
Why the other options are wrong
- A. An imbalanced flywheel would cause a significant, RPM-dependent vibration felt throughout the engine and chassis.
- B. A bent crankshaft would cause a severe, engine-wide vibration that is very RPM-dependent and usually indicates catastrophic failure.
- D. Worn main bearings typically produce a deep 'thump' or 'knock' from the bottom of the engine, usually RPM-dependent.
Engine Noise/Vibration - Accessory
Diagnosing engine noises or vibrations that originate from engine-driven accessories or their mounting components.
- Localized noise/vibration points to specific accessory.
- Use a stethoscope to pinpoint the source.
- Often not directly related to engine internal health.
Memory trick: Listen close, the accessory's dose.