X3 Decelerates Agressively at Random

[X3NShambles]

Please help! Over the last few months, I've intermittently experienced a dangerous and unexpected deceleration while driving my 2016 X3 xDrive 35i, usually occurring somewhere between 30-45 mph. It happens randomly, maybe once every week or two. The first indication is an inability to accelerate (despite being nowhere near the redline -- the tach showed approximately 3k RPM @30mph), then when releasing the accelerator it feels like a serious downshift slowing agressively towards a stop. Once it starts slowing down, I'm not able to get back up to speed again.

Dealership said they couldn't find anything wrong with the transmission, and sent it home, despite a transmission message (message cleared and never re-appeared). I brought it back in thinking if they cleared the transmission then maybe it's the brakes... problem occurred again on my way into the shop and they confirmed that I had an intermittently sticking caliper, so I had them replace the brakes. Problem is still occurring.

The only "quick fix" is to pull over on the side of the road and then power cycle the car. Happens in both Econ and Sport modes. No indication that this is any form of limp mode, and there have been no dashboard indications of a malfunction.

If this happens again on the wrong day with a tailgater, it will become a very bad day. What do you think is the problem/fix... transmission? computer? I don't know what to do next, and the dealership doesn't seem to capable of seriously troubleshooting the issue (they have apparently not been able to replicate the conditions).

[328 Power 04]

Immediately after this happens, possibly before even cycling ignition, I'm curious of any errors from all the modules in the car.

And then also after cycling ignition.

[X3NShambles]

Thanks for your response! There aren't any error messages visible on the dash or screen after that happens, either before or after power cycle.

[bmwdirtracer]

This sounds suspiciously like an issue I just faced with a 2017 435i. Does the symptom have anything to do with rain? How closely have you followed maintenance, specifically brake flushes?

You might want to try unplugging the ABS module, and seeing whether the symptom recurs with it unplugged.

When Abel asked for "errors from all modules in the car", he was not referring to warnings on the dashboard, but stored codes in every computer. You'll need a BMW-capable scan tool for this, not just an OBD code reader.

[X3NShambles]

Thanks for your response, Chris. Yeah, I've definitely kept up with scheduled maintenance, and the dealership just did a brake flush last week when they replaced the sticking caliper.

Happened again today just after startup... noticed that it felt strange and was unable to accelerate beyond 30mph, then once again aggressively decelerated towards a full stop upon releasing the pedal. The problem remained after the first power cycle on the side of the road, but then reset following a second power cycle.

I feel like there's also a faint an odor when this occurs, but it's fleeting and I haven't been able to identify it.

[bmwdirtracer]

Unplug the ABS module, and drive the car.

There is an brake auto-dry "feature" on your car. I have recently fixed a BMW that was malfunctioning something like yours. IF unplugging the ABS module prevents the issue, I can suggest a fix.

[X3NShambles]

Okay thanks Chris, I'll try that. Just to be clear, right now you're just suggesting that I unplug the electrical connection to the ABS module and then temporarily test drive without it connected to see if the problem re-occurs?

[bmwdirtracer]

Yes. You WILL see all sorts of warning lights, and you WILL have all sorts of trouble codes set which will have to be erased. But, if the problem does not recur when the ABS is unplugged, we've narrowed it down to an ABS issue for which I have a possible, not-too-expensive solution.

But again, you will set codes in the ABS computer and other computers which will have to be cleared with a BMW-capable scan tool.

[328 Power 04]

If you unplug the ABS, I'm fairly certain the car will be in limp mode, so expect that.

[jawj52]

Hi,
Brand new user. I'm having the same/similar problem with my 2017 X3. The car will randomly apply the rear brakes causing a big drag speed wise. So far the solution has been to pull over and cycle the parking brake on and off a few times. That seems to correct it until the next time. I noticed in this thread that one member alluded to an inexpensive fix. I would appreciate knowing just what that repair entails.
Thanks much.
Joe-new guy

[bmwdirtracer]

Welcome to the forum.

That particular solution concerned the FRONT brakes, which have a self-drying feature, and referred to a grossly under-maintained car. (The brake fluid needs to be flushed every 2 years).

I'd recommend taking your car to a BMW specialist pro, who can verify your suspicions on a lift, check for applicable codes in the ABS/DSC, and find the culprit.