Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisHard
A vehicle with a V6 engine develops a noticeable vibration that is present only at certain engine RPMs, typically between 1500-2000 RPM. Below and above this range, the engine runs smoothly. The vibration is felt mostly through the shifter and often causes a slight buzz in the dashboard. Which of the following is the MOST likely cause?
- AA misfiring cylinder.
- BA sticking engine valve.
- CAn imbalanced torque converter.
- DA worn crankshaft main bearing.
Show answer & explanationAnswer & explanation
Correct answer: C. An imbalanced torque converter.
A vibration that occurs only at specific RPM ranges, especially when felt through the shifter, is highly indicative of an imbalanced rotating component in the drivetrain. The torque converter, being a large rotating mass connected directly to the engine and transmission, is a common culprit for such RPM-specific vibrations, particularly if the vibration is felt through the drivetrain components like the shifter.
Why the other options are wrong
- A. A misfiring cylinder would cause a rough idle, power loss, and a noticeable engine shake that is usually present at all RPMs where the misfire occurs, not just a narrow band.
- B. A sticking engine valve would typically cause a misfire, rough idle, and a fluctuating or low vacuum reading, not a specific RPM-range vibration.
- D. A worn crankshaft main bearing would cause a deep, rhythmic 'knock' or 'thump' that is present across a wider RPM range, often worsening with RPM, rather than being limited to a narrow band.
RPM-Specific Vibration
A vibration that is only present or significantly worse at certain engine RPMs, often indicating a resonant imbalance in a rotating component.
- Commonly caused by torque converter imbalance.
- Can also be caused by driveshaft/flexplate issues (less common for specific engine RPM vs speed).
- Requires careful diagnosis to differentiate from engine misfires or other component failures.
Memory trick: If the buzz hits a certain tune, torque converter is the reason soon!