2006 M45 Sport - my serpentine belt tensioner is dancing. It looks like the outside ring of the balancer is out of round or off-center from the hub slightly. I took the belt off and don’t feel anything loose. Anyone ever seen anything like that before?

Yes, it could happen if the rubber between the harmonic balancer and the pulley is damaged.
https://forums.nicoclub.com/post6705747.html

Do you have pictures/video of your situation?

Thanks for the link, Ed! Mine looks similar to the video he posted just not as pronounced. The pulley is $385 online. I have a 1200lb impact and a large compressor but I don’t think I’ll have the room to get to it without removing the radiator and condenser. Ugh. I really don’t want to know how much this is going to cost. I just got done doing the idler, all tensioner bearings, the belts, the radiator hoses, the valve cover gaskets, the spark plugs, the power steering hoses, the water inlet, and the coolant. I started this as a kind of tune up but every time I look down there’s another issue.

Well, I’m definitely not going to make any money as a Youtuber. Sorry for the angle. If there is a way to change it, I missed it.

Anyway, I don’t think you can even see the harmonic balancer off-center wobble in this video. I was hoping to capture the small jump that the tensioner does but I think this is the beginning stages and it’s hard to see in video. I didn’t want to drop my very-expensive phone into my drive belt. I have made the executive decision, considering cost versus likelihood of catastrophic failure, and decided to just monitor this situation for a while.

By the way, the ring gear stopper is absolutely NOWHERE to be found. I think you would have to kidnap an Infiniti tech.

https://youtu.be/aO-kTGz4Nh4

From my point of view, the crankshaft pulley doesn't look like it's wobbling.

I couldn't make it out in the video, but is your tensioner making any chattering noise? Some movement is normal (I'll try to record my engine tensioners), but if you are concerned it's best to keep an eye on it.

Bad belts, bad pulleys, bad bearings or misalignment can cause this kind of damage, and likewise, any of those symptoms can cause the other symptoms if left unaddressed long enough.

The tensioner doesn’t chatter since I replaced the serpentine belt with the correct workable length of belt at 76.3 inches, I believe. The tensioner rides right on the middle mark. It does jump a little, though. I have just replaced all belts, the idler pulley with the OEM part, and all bearings in the two tensioners. That’s the reason I noticed it. It’s so slight that it is hard to see on the video but I can watch one side of the harmonic balancer with respect to the engine block and see it move off-center as it spins. It’s so slight that I’m going to keep an eye on it for the next couple of thousand miles and see if there is a change, instead of rushing to spend $1000 on a pulley change.

Your bearing part numbers helped me a lot, Ed. Thanks for that. The idler was a catastrophic failure so I had to buy the whole thing from my dealer.