That may be the balance shaft on the motor.
There are two gears to this part. One is directly behind the upper timing chain gear. The other is just above that. If they are worn, this can cause a vibration. It throws the timing off and that may rock left to right as the engine.
With the mileage on your car, it is about time to replace the timing chain. This would make it easy to replace the balance shaft assenmley at the same time.
The only other thing I can think of is a bad cylinder.
To check the compression crank the engine over at least 5 and record the highest reading. Repeat the procedure for all cylinders using aproximately the same number of revolutions. The lowest compression should not be less than 75% of the highest compression reading.
Compression should not be below 100 PSI.
You can usually borrow or rent a compresion test gauge at an Advance auto or auto parts store. It will have adapters to fit the sprak plug holes on your engine.
That about covers it.
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Tuesday, September 27th, 2011 AT 1:41 PM