I'm really clueless with this. It's not a big deal but it drives me nuts nonetheless.

This is on a M3.8.3 cable throttle system.

After a cold start, for a minute or so I get a high idle at around 1500-1600 rpm. The weird thing is that it seems to drop to normal (~1100) when coming to a complete stop, but if the car is moving even so slowly, the idle increases again (does not matter if with clutch disengaged or in neutral).
When warmed up, the idle is sometimes spot on, but other times it is slightly higher. But it is always stable, without misfires/hunting etc.
I also get inconsistent engine braking (overrun) behavior, sometimes it transitions instantly, sometimes after a while and sometimes never.
Except than this, the car works perfectly fine. There are no DTCs.

What I tried:
-intake manifold gasket, TB gasket
-cleaning and adapting the TB
-throttle cable does not stick
-MAF
-checked all vacuum lines and also PCV plumbing, replaced suspicious ones but there were no obvious cracks
-tried blocking vacuum lines to charcoal canister and to PCV without effect
-tried another ECU
-coolant temperature sensor works properly


The only thing I can think of which I did not test are vacuum check valves and PCV valve on the intake, but this should be ruled out by blocking the PCV vacuum line.

Could the idle regulation mechanism in the TB be shot? I removed the cover and everything looked nice and clean inside, no scratches on the resistive tracks.

To me this looks like metered air which is taking a different route to the manifold than through the TB, but I cannot figure it out. Or bad TB itself...


Thanks for any help  

"The only thing I can think of which I did not test are vacuum check valves and PCV valve on the intake, but this should be ruled out by blocking the PCV vacuum line."

The small air vent valve on the lower engine located on the T-hose may be broken and allow air to enter the intake. Also check for leaks in the downpipe joints as this will distort the lambda reading...

The small check valve works like this: when the throttle is closed, so when you idle/decelerate, you suck in some of the low engine gases, and in positive gear it is closed. The entire PCV system must be in good condition, this amount of air is calculated. But not enough to make it go up to 1500 rpm...there must be something else, leaks.


Check with vcds the lambda values...rpm...temperatures...idle speed...

And this is why on a turbocharged car the first test you do is a boost pressure test.
Remove inlet hose from the turbo, add a bung with a valve and pressurize to 0.5-1 bar or so.

You have boost leaks. And no you are not going to find them by sucking on things.