A harmonic balancer will not cause a vibration that severe. If it did, it would've probably broken the crankshaft years ago. Also, any check engine light? If so, scan comp and send me the codes. (A faulty balancer would get worse at higher engine speeds, not just sitting idle. That in itself eliminates that possibility)
Check for a misfire by doing a drop test on each cylinder.
Sunday, February 27th, 2011 AT 3:44 AM