Recently I took my other half 330i to get inspected. There was no engine light on dash for what ever reason and it failed the epa part of test. It had two codes,
one was adaption limit # 3 & 4 cylinder and a 02 sensor. After calling my bmw friend I did a few things and it was still throwing the two codes. So I finally took it to a independent bmw shop and told them what was going on.
They said most likely a vacuum leak somewhere.
Here's the whole point to this, they did a ^^^^smoke test^^^^ and it showed smoke way down below the oil separator. It was a cracked rubber hose. Replaced hose(500.00 $ later) and both codes cleared and passed inspection.
With these mini trucks Ive seen and heard many a vacuum leak that made the trucks have a rough idle. Its finding that vacuum leak that makes you beat your head against the wall.
You can buy the smoke test machine for 600 to 1000.
I think a little peanut oil( is what the store bought machines use) and a little heat source you could build one???
Kill two birds with one stone, use it on the mini and use it on your regular vehicle.