web page Every time the cylinders fire, torque is imparted to the crankshaft. The crankshaft deflects under this torque, which sets up vibrations when the torque is released. At certain engine speeds the torques imparted by the cylinders are in synch with the vibrations in the crankshaft, which results in a phenomenon called resonance. This resonance causes stress beyond what the crankshaft can withstand, resulting in crankshaft failure.
To prevent this vibration, a harmonic balancer is attached to the front part of the crankshaft. The damper is composed of two elements: a mass and an energy dissipating element. The mass resists the acceleration of the vibration and the energy dissipating (rubber/clutch/fluid) element absorbs the vibrations.
Additionally the energy transferred from the piston to the crankshaft can induce as much as 2 degrees of twist in the crankshaft, which has many follow-on effects on all engine elements that require adequate timing such as valve opening, cam timing, ignition timing etc.
If your running at a resonate frequency of the system with a bad damper you can damage the main bearings. The higher degree of vibration will increase spark scatter and could thow a misfire code.
If you had a bad damper and a non firing plug that would result in a much larger impulse variation. If that happens at resonant speed of the crankshaft it could generate very high loads on the damper and the mains.
A failed damper can damage your mains bearings. A misfiring plug increases the loading on the damper and can shorten its life.
The hard part would be showing cause and effect of a disconencted plug to a failed damper. The damper may have been failing all on its own.
There is no clear answer on the effect of a bad damper, it will depend on the rpm that the motor was run at, oil film thickness and the loading. If you never hit the resonant frequency it wont matter. If you have a bad damper and your at the resonant frequency it could be a matter of seconds for a crank to break and or start damaging your mains.
Good read:
web page 1: Yes the ballancer can fail over time, the rubber can get hard, slip or lose it properties.
2: Too many variables to be able to provide an answer.
Kevin
