Car revving when at a standstill

JaMarcusHustle

Hi,

I recently had a new alternator put in to my BMW e90 as the old one was faulty. A slight oil leak had contaminated it. Getting the oil leak fixed this week.

Since I got the car back from the garage after the alternator was replaced, I've noticed that whenever I'm stopped at lights or pulled in somewhere with the car still running, it is revving ever so slightly. You can both hear the revving and see it shooting up on rev counter. Here's a quick video from the other night - sometimes it goes higher on the tachometre than it does in this video:

https://www.youtube.com/watch?v=JLzD8NHsW-c

This happens both when I'm in gear and when I'm in neutral. I'll be asking about it when I drop it back in this week, but anybody any ideas in the meantime? I don't see how it could be linked to the alternator being replaced, is that just coincidental timing?

Thanks for any advice.

cylor

Saw a similar issue in an E46 before - twitchy tachometer at idle, but otherwise drives ok - in that case it was because of a broken DISA valve / adjuster unit.

Stoolbend

It's possible there's a vacuum pipe left off or has just split after being moved while replacing the alternator.

Cee-Jay-Cee

It's called 'hunting' and usual suspect is a split vacuum pipe somewhere on the inlet manifold.