I recently had a Wavetrac limited-slip diff installed on my 6-speed M240i, which included a ratio change from the stock 3.08 to 3.46. Although the car drives fantastically after startup, I've ran into a peculiar issue soon after the car starts. It should be noted that the car is bone stock besides the diff and an intake, and there's no check-engine light or warnings from iDrive. I always drive in DSC OFF, so it's likely not traction control related.
When the car is first started, cold or not, I lose acceleration in-gear after about a second. The turbo will start spooling and the car will accelerate normally for a second, but immediately cut power after, even if the gas pedal is pressed to the floor. The car will still accelerate, but at a painfully slow rate, almost unsafely slow. If I let off the gas and floor it again, you can feel a momentary blip of power before it cuts again. If you look in Sport Displays on iDrive, it shows a very low power number, indicated the ECU knows exactly what it's doing. If I shift gears, whether upshifting or downshifting, the car will behave normally again for a second before the same behavior returns. I'm not sure exactly what causes the behavior to go away, but the car will suddenly begin behaving exactly as expected between 10-60 seconds after beginning a drive.
I called my mechanic who installed the diff, and described the behavior to him. He is unsure what caused this, as he said the engine systems were untouched, which makes sense. Our leading theory is that the ECU is confused by the wheel speed not matching the expected RPM due to the new axle ratio, and cutting power in an attempt to diagnose the issue. I find this very likely, as I've noticed the ECU cutting power if I have excessive clutch slip during an aggresive gear shift. Perhaps the car is trying to prevent what it perceives as clutch slip? My other theory is that it has to do with some sensors in the suspension. To install the diff, the shop had to disconnect the
entire rear multilink suspension system, and they replaced a bent front control arm unrelated to the diff.
I'd love to hear from any other forum members who may have had this issue, and how they fixed it if they managed to. I know that, even among owners who swap in a limited-slip diff, the ratio change is uncommon, so I don't expect to find any solid conclusions.
EDIT: a video link to demonstrate the issue.
https://www.youtube.com/shorts/EnszNYXpfBY