It doesn't sound like limp mode sounds like brakes stuck on.

If you can get to the bolts that hold the MC to the booster loosen them a little and it should relieve the pressure in the line vs. crack ab hydraulic line.

You can pull the vaccum line off the booster to prevent it from "taking over" for testing but be prepared to use the parking brake to stop. Does the pre lci e70 ise the electric parking brake?

When the brakes are held the DME will cut power to the engine. As you slow down, it will downshift and once stopped you will likely not be able to move or just crawl.

The symptoms you describe m exactly match booster over zealous.

I've seen a couple times and always resolved.

I'm not sure you can pull the check valve for the booster that would be the easiest way to relax the booster to test it.

High confidence it's the booster over acting.

Loosen the MC mounting a little if you can, just 1/16 to 3/32" else check valve.

If MC is a little loose and maybe use a small shim like washer to hold it away from the booster it should immediately behave.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)