Deceleration Jitter/Stutter

[Arsonism]

Ever since the purchase of my Coupe I've noticed that on deceleration there's a strange sensation. If you let off the gas hard and with little transition you'll feel the car jerk and stutter for a half second to a second before you actually begin decelerating. The harshness of this stutter is proportional to the rate of the transition and is dramatically increased in sport mode and higher RPMs. I'm not entirely sure if this is just a normal sensation for the S54 power train or if something is starting to break loose. I also feel like it could be something simple like a transmission mount that I've seen several people have gone forward and replaced.

Just looking for more diag intel and common known issues.

[The HACK]

Hard to tell from your description, but I think it's normal. Unlike older cars with mechanical throttle plates, cars with electronic throttle are programmed to keep the throttle plate open to lower NOx emissions and lower fuel consumption. The end result is most cars won't decelerate immediately when you get off the throttle, and you'll see the RPM stay high for a split second before coming down. It's most evident between shifts.

On the M, unlike it's single throttle plate brethren, has six individual throttle openings for each cylinder. While the electronic controls can open and shut those throttle openings very fast, it's still six separate throttle all closing at different times. So unlike a normal BMW that transition from open to closed throttle isn't as smooth and you'll get a short, momentary "stutter" typical with cars that has individual throttle openings for each cylinder.

Of course, this is all conjecture. Only way to really know if you have a problem or not is to drive another S54 equipped car to compare to yours.

[Arsonism]

Thanks Hack,

There is another gentlemen with a Moupe in my area and maybe I can ride along with him. I hadn't thought about the actual disengagement of the pedal to the close of the various throttle openings.

I suppose my concern was "chatter" from the differential or that there was a disconnect in the transfer of power upon deceleration. The whole car will literally jerk, almost lunge when you just let off the gas and I suppose that's my concern. If you let off smoothly this disappears so I feel like it must be something that you've described above.

In my head I feel like this would lead directly to some of the various mount locations to better stabilize the power train, just didn't know if this was a symptom or a standard sensation. Keep in mind I come from a Z3 (non M).

[bignosejim]

Another possibility may be loose motor mount bracket bolts on the passenger side of the engine. This is/was a common issue on these cars. You can see the bottom forward bolt by looking in from the wheel well. It's pretty easy to check that particular bolt, but the rest of them are not so easy.

[Arsonism]

Thank you Jim!!! I will be looking at this when I get under the car next (when RPI's arrive).

[Arsonism]

I wound up discovering the primary cause of this unexpectedly as it was greatly decreased when I changed the mufflers (to RPI) over a weekend or two ago. I suppose the baffling was deteriorating in the original cans.