TheOtherDave

I have a 2021 C4S 7MT that has a vibration issue.

When at partial throttle passing from 75-80MPH and really getting bad at 85MPH-what ever speed above this you care to go to
there is a vibration that starts and gets bad enough to feel it in your butt and in the steering wheel.

Initially I figured it was the wheels balance as the stock Pirelli's were near end of life. But a new set of tires and a fresh balance
hasn't fixed it.

Through further testing I have determined
1. It is related to engine load. Part to full throttle causes it. No throttle or push in the clutch and it goes away.
2. It is not related to gear. I have duplicated it in 4-7th.
3. It is not related to road surface. I have duplicated on smooth as glass and rough roads.
4. Doesn't seem to be related to temperature.

Any thoughts or ideas on what might be causing this? I am well equipped to look at any parts/areas
that people think might be suspect.

I am trying to schedule a service appt with my local Pcar dealership (car is still under warranty) but they claim
they are out into Aug (oh and they are a bunch of idiots).

Thanks,
Dave

drcollie

You've done the easy stuff and have a good diagnostic procedure. The "continuing under load only" part rules out a lot of items like a bent wheel or bad control arm. I think I would look at the dynamic engine mounts which your "S" has standard, they are known problem areas that would be load-specific. Other than that a bad half-shaft or bearing?

TheOtherDave

Those are (mostly) my thoughts as well. I also wonder about the drive shaft to the front wheels or maybe a front differential mount.

Since the car is still under warranty I tried to book an appt with my local Pcar dealer online....they show no openings through August.
I may have to make the trek from Northern Colorado to Lakewood to get to an Pcar dealer that actually has techs working for it.