Limp mode triggered by braking system?

My car went into limp mode (maximum 2k revs, really sluggish throttle response and 1100 idle speed) twice in the last few days, each time within a mile from cold start.

The iDrive displayed an amber engine management warning light and said that driving moderately was OK but I should take it to a dealer (obviously...).

I tried charging the battery for 48 hours between incident 1 and 2.

BMW Assist tech guy came to my home today, and read the limp mode messages from the OBD, but the car started and ran (for about 4 miles) without problem.

He did say that the brake vacuum system (pump, sensor or seals) was indicating a fault however (and the pedal did feel a bit "soft").

I am taking it into the dealer tomorrow for repair.

Just wondering if anyone else has experienced this?

There is a pressure sensor under the brake fluid reservoir that can go bad if somebody spills brake fluid on it during a service.

If it genuinely is a brake system problem, surely it would make sense for the iDrive error message to state this, explicitly?

Tony B wrote

My car went into limp mode (maximum 2k revs, really sluggish throttle response and 1100 idle speed) twice in the last few days, each time within a mile from cold start.

The iDrive displayed an amber engine management warning light and said that driving moderately was OK but I should take it to a dealer (obviously...).

I tried charging the battery for 48 hours between incident 1 and 2.

BMW Assist tech guy came to my home today, and read the limp mode messages from the OBD, but the car started and ran (for about 4 miles) without problem.

He did say that the brake vacuum system (pump, sensor or seals) was indicating a fault however (and the pedal did feel a bit "soft").

I am taking it into the dealer tomorrow for repair.

Just wondering if anyone else has experienced this?

OP, did you recently have a brake flush?

I experienced the same issues when I had my brake fluid flushed and the indy shop haphazardly overfilled the reservoir and fluid runoff malfunctioned the brake fluid reservoir.

You should hear your a whining noise coming from the footwell. The pump staying on longer than it should.

I was able to prove my case and the shop remedied the issue. At first they wanted me to pay to address the issue.

SpacegrayMe90 wrote

Tony B wrote

My car went into limp mode (maximum 2k revs, really sluggish throttle response and 1100 idle speed) twice in the last few days, each time within a mile from cold start.

The iDrive displayed an amber engine management warning light and said that driving moderately was OK but I should take it to a dealer (obviously...).

I tried charging the battery for 48 hours between incident 1 and 2.

BMW Assist tech guy came to my home today, and read the limp mode messages from the OBD, but the car started and ran (for about 4 miles) without problem.

He did say that the brake vacuum system (pump, sensor or seals) was indicating a fault however (and the pedal did feel a bit "soft").

I am taking it into the dealer tomorrow for repair.

Just wondering if anyone else has experienced this?

OP, did you recently have a brake flush?

I experienced the same issues when I had my brake fluid flushed and the indy shop haphazardly overfilled the reservoir and fluid runoff malfunctioned the brake fluid reservoir.

You should hear your a whining noise coming from the footwell. The pump staying on longer than it should.

I was able to prove my case and the shop remedied the issue. At first they wanted me to pay to address the issue.

+1👆🏼Very common.

Somewhat common for this sensor to fail and cause limp mode. Failure is almost all ways due to fluid contamination.

If this sensor is the fault, braking ability is not reduced as vacuum is not reduced, only its data stream to the DME is faulty.

iDrive notifications are exceptionally vague, they never identify anything useful.

deansbimmer wrote

Somewhat common for this sensor to fail and cause limp mode. Failure is almost all ways due to fluid contamination.

If this sensor is the fault, braking ability is not reduced as vacuum is not reduced, only its data stream to the DME is faulty.

iDrive notifications are exceptionally vague, they never identify anything useful.

That has been my experience with iDrive diagnostics as well. Slightly less vague than a 50 dollar autel.

Easy DIY - had the same problem in May

Replacing Brake Vacuum Pressure Sensor https://www.m3post.com/forums/showthread.php?t=1338052

SpacegrayMe90 wrote

OP, did you recently have a brake flush?

I experienced the same issues when I had my brake fluid flushed and the indy shop haphazardly overfilled the reservoir and fluid runoff malfunctioned the brake fluid reservoir.

You should hear your a whining noise coming from the footwell. The pump staying on longer than it should.

I was able to prove my case and the shop remedied the issue. At first they wanted me to pay to address the issue.

Hmmmm....

Brake fluid change about 6 weeks ago.....by the Main Dealer.
I have not heard any excess pump noise, nor noticed the pump running for longer than normal (TBH - I am not sure that I have noticed any pump operating noise anyway).

If it is bad work by the dealer, I think I might well be refusing to pay the £100 excess on the warranty claim.

deansbimmer wrote

Somewhat common for this sensor to fail and cause limp mode. Failure is almost all ways due to fluid contamination.

If this sensor is the fault, braking ability is not reduced as vacuum is not reduced, only its data stream to the DME is faulty.

iDrive notifications are exceptionally vague, they never identify anything useful.

I would say that brake pedal feel has been quite "soft" recently. I had to brake quite hard to avoid rear-ending someone who changed his mind during an overtake. Pedal went a long way down and I was surprised that the ABS did not activate.

Dealer left me a voicemail this morning, stating that it is not a faulty sensor, but there is a vacuum leak, which they are still trying to locate.