SMB, in theory, you are right. In practice, not only does the amplitude of the stresses increase with engine speed, so does the frequency. The engine has mass so that even though the stresses are greater, the mass of the engine will simply not allow the engine to shake that fast or that far. As a result, the vibrations max out at one RPM and can be felt over a small range of engine speeds.
Some people remove the balance shafts on purpose and just live with the vibrations. The 2.4 liter version of this engine does not have them. An out of phase (mistimed) balance shaft would be worse than no balance shaft. Out Honda uses balance shafts and I suspect that the rear shaft was not timed correctly when the first timing belt was replaced. It is very tricky to get it right. after the second timing belt change, the engine has been a lot smoother. BTW, it vibrated worse at around 2200 rpm, but the vibration was mostly heard rather than felt so the balance shaft couldn’t have been very far off.
I am not saying that I agree with the balance shaft diagnosis, I too am very doubtful, but I can see a case for it.
I just had another though, since the balance shafts have a gear on the front of each one, and the gear is probably held in place with a bolt and a pin, if the bolt was left loose at the factory, eventually the pin would break and the gear would freewheel. That would not make a lot of noise.
Maybe a call to Nissan customer service might be in order, although they don’t seem to have too good of a reputation, considering the way they handled the oil consumption issue with this engine in 05/06.
No disrespect, Keith, but I think you’re “reaching”. I think this is a simple case of an engine vibration through a specific operating range and the dealer just didn;t want to be bothered and pulled an excuse out of their…hat.
If I learned that the dealer had actually used diagnostic equipment and techniques I could be persuaded to change my mind. But I haven’t read anything to suggest that.
TSM, yes I am “reaching” to make a case for this, but what I have said is factual. As I said on more than one post of this thread, I do not agree with the diagnosis, I can just find a case for it. I would really suspect the vibration dampener.
A new engine would be the almost 100% guarantee of eliminating the vibration, unless it is the vibration dampener. If the OP bought the new engine and it didn’t eliminate the vibration because it turned out to be a bad dampener, would the dealer put her original engine back in and give her a full refund?
I tend to agree with mountainbike about a possible CV joint or engine performance problem possibly being the cause of this. The time to check the joints would have been while the transmission was out; assuming someone there knew what they were looking for.
An ignition miss, MAF fault, etc. can also do funny things at certain RPMs.
No way on Earth would I authorize 6 grand for a wild guess fishing expedition that would likely improve nothing and the balance shaft diagnosis is still extremely suspect in my opinion.