Intake issues (MAF, fuel trim, etc - P1141, P1127, P1129) [SOLVED FOR NOW]
 

Looking for suggestions... I've had this '98 A4 Avant 2.8 wagon for about nine years now, and the CEL has been intermittent the whole time. California requires a smog check every other year; one year it just needed a gas cap, in 2014 I was able to reset with VAG COM and the CEL stayed out for many months. But lately the car has a rough idle from time to time, occasionally will have a big hesitation or stumble under load, some days will repeatedly stall at idle when cold.

And now that I'm finally looking into it, I'm getting this fault instantly whenever I clear the codes for the engine:
The car appeared to be idling normally. MAF has been replaced twice while I've had the car, current part shows "Made in Japan" and was installed in 2011. When I switched off and unplugged it, there was no difference in the idle on restart. The first time I cleared codes with it unplugged, the car died instantly; subsequent times, the revs dropped and it nearly stalled, but recovered and idled normally. FWIW.

I've ordered vacuum line because it's cheap. I've never smoke tested vacuum lines, seems simpler to just replace since I've no record of it having been done but right now it's constantly raining. If/when I catch it idling rough again consistently, I'll try unplugging the MAF again.

Thanks in advance for your comments!

Before doing anything with the vacuum lines, I decided to give cleaning the MAF sensor a try. Lo and behold, everything but the16826/P0442 "EVAP: small leak" problem cleared up.

The rubber seal on the gas cap looked and felt good, but was ~5 years old. I put some silicone lube on it and was able to get 70-80 miles before the "small leak" triggered again. Cleared codes, ran it for ~50 miles, passed the smog test, and the "small leak" popped again about 10 miles later.

But it was enough to get the registration renewal completed. Actual emissions figures on this car are so low compared to legal limits as to be laughable, it's just the damned CEL that causes problems.