ASE A1 Engine RepairGeneral Engine DiagnosisMedium
A customer complains of a persistent vibration that is most noticeable at idle and low RPMs, and seems to smooth out as engine speed increases. The vibration is felt throughout the vehicle. Which of the following is the most likely cause?
- AAn unbalanced tire
- BA driveshaft imbalance
- CA worn engine mount
- DA sticky brake caliper
Show answer & explanationAnswer & explanation
Correct answer: C. A worn engine mount
Vibrations most noticeable at idle or low RPMs, and felt throughout the vehicle, are characteristic of worn or broken engine mounts, which fail to isolate engine vibrations from the chassis.
Why the other options are wrong
- A. An unbalanced tire typically causes vibrations that increase with vehicle speed, often felt in the steering wheel or seat, not primarily at idle.
- B. A driveshaft imbalance typically causes vibrations that increase with vehicle speed, felt in the floor or seat, not at idle.
- D. A sticky brake caliper would cause a pulling sensation, possibly a burning smell, and reduced fuel economy, but not a general engine vibration at idle.
Worn Engine Mounts
Engine mounts isolate engine vibrations from the vehicle chassis. When worn, they allow excessive engine movement and transmit vibrations, especially at idle or under load.
- Isolate engine vibrations
- Worn mounts cause vibrations at idle/low RPM
- Can cause clunking sounds during shifting or acceleration
Memory trick: Idle shake? Engine mount's brake!