Idle Speed Change with Brake-- Normal?

[Beemdog]

So I've noticed when sitting at a stoplight with my 2011 DCT, if I release the brake the idle moves up about 150 rpm or so. This is repeatable behavior. Is this a normal condition for a DCT-equipped M3? Seems odd, like the car is anticipating that it is about to be set into motion.

[MikeD]

Yes, I think this is normal. Google this, it happens with most cars, related to brake booster, it uses vacuum from engine intake.

[Beemdog]

I was thinking brake booster too, but I haven't seen this on my other (non-BMW) cars. Still suspicious. Anyone else have this symptom?

[MikeD]

Could signal a leak if the fluctuations are slow

[AUTiger13]

Mine does the exact same thing. It has done this ever since I've had the car and have passed it off as normal, unless I am told otherwise.

[Beemdog]

Originally Posted by MikeD

Could signal a leak if the fluctuations are slow

It acts like a switch. Brake on, idle is at 600 RPM (if I recall)). Brake off, it goes up to 700 or 750-ish (need to take a closer look). I was bored sitting at a stoplight and noticed this. Car drives fine. Seems odd. I would imagine there is closed-loop control by the ECU over the idle speed, so it is actively trying to achieve a target RPM. Why would the target change with the brake engaged?

[Beemdog]

Originally Posted by AUTiger13

Mine does the exact same thing. It has done this ever since I've had the car and have passed it off as normal, unless I am told otherwise.

Hmmm, good input, I'm thinking this could be in the design then.

Any engine gurus know why they would do this?

[AUTiger13]

My only guess is that since the brake booster runs off of vacuum, when the brakes are depressed, some vacuum is diverted or used to increase the pressure on the brakes. When stopped at a light, this decrease in vacuum is apparent to us as a slight decrease in rpm.

That was a just a shot in the dark, so take it for what it's worth.

[GB///M]

I always wondered this too because it happens with my car as well. I got my car new so just thinking it's normal.

[Beemdog]

I played with this a little more and it turns out the idle speed doesn't change with the brake pedal depressed/not depressed when the DCT is in Neutral. So it looks like this is a programmed in feature... for what purpose, I'm not sure!