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Let me share my 2 cents.
I have been following this forum (and others) for the past 6 years.
The first time I read about the vibration, was in one of Jake Raby posts about the potential reasons for the IMSB failure.
If to be honest, no one really convinced me with the true reason for the failure. Although there are a liot of theories out there, there is no logic explanation (to me) why some fail and some don't.
The only theory that made a sense to me, is the one presented by Jake, talking about the vibration at the 3000 RPM range.
If I remember correctly, he mentioned that the flywheel is acting as a harmonic balancer, and there is a natural vibration due to the tension on the IMS to crank chain.
He also implied that low mileage cars are more prone to failure under the assumption that they are not driven hard enough. Also explain some documented cases of multiple failures with the same driver, to suggest the way you drive your car contributes to the IMSB failure.
Up to here, these are words of my memory, so I might not be 100% accurate (and I apologize for that) with Mr. Raby words.
To speak from my personal experience, I can tell you that there is a significant vibration in the 2700-3200 RPM range. It is mostly noticeable on deceleration. (Meaning when the engine RPM drops from 3200 to 2700) and RPM fluctuation.
On my ex 99 base, the flywheel was completely shot at 160K miles.
I could clearly feel the vibration at the above mentioned RPM range, and it was gone, as soon as I installed a new flywheel, just before e I sold the car.
Since then (and if to be honest also before) I try to keep the RPM on the 3000-4000.
It is also more fun, although less fuel efficient.
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