Car: 2012 Audi A4, 235,xxx miles, CAEB (2.0T) engine

TLDR: Car is running fine, P2279 (leak in air intake) sticks, P2187 (lean at idle) is intermittent. Points to a vacuum leak, but couldn't find anything smoke testing. I've done a number of tests, and I'm currently stumped.

Longer description:
I have two fault codes: P2279 "Leak in air intake", which is constant, and P2187 "Bank 1: System lean at idle", which is intermittent. This one seems to pop up at stoplights. There are no issues with driveability or the feel of the engine - it idles smoothly, and makes power to redline. The only time there is an issue is when you clear codes, resetting fuel trims - this causes a somewhat rough, cycling idle until the fuel trims are learned.

Visually inspected all of the boost piping and vacuum lines I could see - no cracks, nothing loose.

Next, I tried clamping off the line from the evap solenoid to the intake manifold. This didn't change anything. I then unplugged it - this got rid of the previously mentioned codes, but threw a new one for the unplugged evap - I was thinking this upset the ECU and prevented it throwing the other codes.

Next, I swapped the PCV. I just put a new OEM Hengst unit on about 5,000 miles ago, and I still had the old one - which had no problems, this was preventative maintenance. This made no difference, and behavior pulling the oil cap off was the same - an initial stumble, then a cyclic chugging. The cap wasn't particularly hard to pull off, and the crankcase vacuum seemed to pulse in and out when I put my hand over it. No whistling or honking.

Next, I cleaned the MAF with MAF cleaner. This didn't fix the issue.

Next, I sprayed the (highly flammable, I tested it) MAF cleaner around all of the vacuum hoses, in/around/under the manifold. I saw no changes in the RPM.

Then, I tested the IAT sensor, sitting cold. It reported 12.0 C, while oil temp reported 9.1 C, coolant reported 11.2 C, ambient reported 1.5 C.

Next, I then did some logging on the drive home from work, and let it idle hot. I uploaded the CSV of this data and a python notebook I used to make some plots.
I am a bit confused by the names of the advanced values from VCDS - chatGPT thinks the long term adaptation is an incorrectly scaled miltiplicative trim. There is "Short term adaptation of mixture formation bank 1", "Long-term fuel trim bank 1 at idle" and "Long term adaptation of mixture formation bank 1". The "long term adaptation" moves frequently between 10% and 45% while driving, and settles at 54% at idle. The "long term fuel trim" stayed pretty constant throughout, moving from 5.95% to 5.77%.
Overall, the STFT stayed between -5% and 5%, though it dropped as low as -15% and went as high as 11%. At idle it is about 0%.
Lambda stayed right around 1.0, obviously jumping to the max during deceleration, and dropping to 0.8 on a WOT pull to redline. Interestingly, during this pull, lambda briefly went very positive right when I put my foot down, then rapidly dropped. At idle, it is a smooth 1.0.
The commanded evap position is 0% on a cold start, cycles between 0% and 100% during driving, and sits at 20% on a hot idle.
IATs were around that 9 C mark throughout the drive, and climbed to 38 when I did a hot idle.
The MAF seems to track the RPM as we drive, and doesn't drop out or do anything super weird. Once the idle dropped to 750 RPM on the cold start, it was reading around 2.9 g/s with IATs of around 10 C. At the end of the drive on the hot idle, it was reading 1.75 g/s around with IATs of around 30-35 C. At WOT and 5,000 RPM, it read around 152 g/s.
Boost behavior looked great, with the actual boost bang on to the specified boost during harder pulls. The lower limit of the actual sensor does seem to be above the specified boost most of the time.

After this logging, I bought a smoke machine, and tested the entire intake system for leaks. I tested the boost hose by the MAF, the boost hose by the throttle body, the PCV hose going into the PCV valve, the PCV hose going into the intake manifold, and the throttle body going into the intake manifold. I did not see a single whisp of smoke anywhere in the engine bay. Following some advice, I opened the oil cap - there was a very faint whisp of smoke that came out. Additionally, the PCV did not let me push smoke through it.
I also checked fuel rail pressure while driving - it seemed fine, from 50-100bar.

Next, I went back and took a closer look at the intake manifold flap mechanism - moved it with my hand, double checked the vacuum lines, and used a mirror to check the bushing for cracks. The mechanism is smooth, there isn't any play, and nothing seems cracked.

Next, I went back to my evap solenoid, and disconnected the line between the solenoid and canister. I tried pushing smoke through the solenoid into the manifold, and it (correctly) wouldn't let me. I also tried blocking it with my finger with the engine running, no change.

Finally, I busted out the smoke machine again, and tried externally spraying the smoke out of the small tip, to see if it got sucked in anywhere with the engine running - didn't see anything.

Things I might still try:
- Blocking off the port on the intake manifold for PCV
- Moving and wiggling the intake runner and watching the fuel trims
- Parts cannon


2012 Audi A4, P2279 and P2187