[Zamx95]

Hey everyone,
So my car acts werid whenever I come to a stop too quickly, the rpm drops around up to 500 and then the engine quickly revs up by itself upto 1k rpm and then stablises around 700 rpm, but during driving normally or driving hard my car is smooth and strong. It's only when I'm idling the rpm fluctuates a little or when I come to a stop. Does anyone know what may cause this? Thanks for the help in advance.

[9352e90]

I have a 2008 X5 3.0si that does the exact same thing. If you have your AC running does it keep it from dropping?

[robthewrench]

Sounds like you have a brake booster vacuum issue. The booster uses engine vacuum to increase the force of the braking power. If the booster has a leak it will pull clean air into the engine vacuum causing the RPM to drop (too lean), the computer sees the RPM drop and compensates by adding fuel (the increase in RPM after the stop). Once stopped, if you are pushing less on the brake pedal, the booster is now working as much and the issue disappears.

Try to duplicate the issue by pushing hard on the brake pedal while idling and in park.

[realtrevor]

My car does the same, it's even worse when the AC is on, in some instances it has stalled the engine. I suspect VANOS solenoids are worn out. I need to get it checked out.

[eyehategod]

Originally Posted by robthewrench

Sounds like you have a brake booster vacuum issue. The booster uses engine vacuum to increase the force of the braking power. If the booster has a leak it will pull clean air into the engine vacuum causing the RPM to drop (too lean), the computer sees the RPM drop and compensates by adding fuel (the increase in RPM after the stop). Once stopped, if you are pushing less on the brake pedal, the booster is now working as much and the issue disappears.

Try to duplicate the issue by pushing hard on the brake pedal while idling and in park.

I'm having the same issue as OP - what do you recommend to fix this? Is it a module that needs replacing, or a vacuum line?

[robthewrench]

Originally Posted by eyehategod

I'm having the same issue as OP - what do you recommend to fix this? Is it a module that needs replacing, or a vacuum line?

Try to duplicate the issue by pressing on the brake pedal hard while idling. If the RPMs drop this will confirm that something in the brake booster is not working correctly.

If this test does not make the RPMs drop, then it is not the brake booster and instead you may have a sensor issue or other vacuum leak.

This test may help:

https://www.youtube.com/watch?v=M2AnK8Q90Lo

Also here is a description of the stalling problem:

https://www.yourmechanic.com/article...-brake-booster

[eyehategod]

Originally Posted by robthewrench

Try to duplicate the issue by pressing on the brake pedal hard while idling. If the RPMs drop this will confirm that something in the brake booster is not working correctly.

If this test does make the RPMs drop, then it is not the brake booster and instead you may have a sensor issue or other vacuum leak.

This test may help:

https://www.youtube.com/watch?v=M2AnK8Q90Lo

Also here is a description of the stalling problem:

https://www.yourmechanic.com/article...-brake-booster

Thanks so much! I'm pretty sure I do have a separate vacuum line issue, I need to replace all of the vacuum lines on mine but haven't been able to find a good diagram for the N55. The only vacuum lines I've noticed on the car (I'm a noob) from working on it were the two that run off of the boost solenoid, but I'm fairly sure there are others?

[juld0zer]

Manual or automatic/DCT?

Our cars don't use manifold vacuum to operate the brake booster. The engine drives a vacuum pump off the chain drive.

So some of the traditional textbook methods of diagnosis cannot be applied here