Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisMedium
A client reports that their vehicle experiences a noticeable, but not severe, vibration that seems to worsen slightly with engine RPM, but is present even at idle. The vibration can be felt through the steering wheel and floorboards. A visual inspection reveals no obvious issues with engine mounts or accessories. Which of the following is the MOST likely cause?
- AAn imbalanced tire or wheel assembly.
- BAn internal engine imbalance.
- CA faulty torque converter.
- DA worn universal joint (U-joint) in the driveshaft.
Show answer & explanationAnswer & explanation
Correct answer: B. An internal engine imbalance.
A vibration that is present at idle and worsens with engine RPM, but not necessarily with vehicle speed, points towards an engine-related issue. If external components like mounts and accessories are ruled out, an internal engine imbalance (e.g., crankshaft, flywheel, harmonic balancer) is a strong suspect. This vibration would be felt throughout the chassis, including the steering wheel and floorboards.
Why the other options are wrong
- A. An imbalanced tire or wheel assembly would cause a vibration that is directly proportional to vehicle speed, usually noticeable at specific speeds (e.g., 45-70 mph), and not at idle.
- C. A faulty torque converter usually causes a vibration or shudder during specific lock-up conditions or gear changes, and might not be present consistently at idle or directly proportional to engine RPM across the range.
- D. A worn U-joint would typically cause a vibration that is more pronounced with vehicle speed, often under acceleration or deceleration, not primarily with engine RPM at idle.
Engine Vibration Diagnosis (Internal Engine)
Diagnosing vibrations caused by rotating components within the engine, such as the crankshaft, flywheel, or harmonic balancer.
- Vibration is directly proportional to engine RPM.
- Present at idle and throughout the RPM range.
- Felt throughout the chassis, steering wheel, and floorboards.
- Requires ruling out external engine components and accessories.
Memory trick: If the shake matches the engine's pace, internal imbalance is the case!