AlexA8V8

Hey fellow OOOO fans, i have a proper head scratcher that i need help with. It's a long-winded story so bare with me. I really need help 'cuz i'm running out of ideas.
So the car is Euro version, A8 4.2L V8 petrol, Quattro from 2000, so a Facelifted model, stock output of 228kw. The engine is AQF / AUW (i believe).


The problem/anomaly and symptoms:
- When coldstarting after a 6+ hours of standing, it will crank up easy, but then rpm's rapidly jump to roughly 2000 and drop to almost zero as if sputtering out repeatedly for 3-5 times, then jump to 1500rpms and hold for 3-5 seconds and slowly glide down to normal idle around 800rmp.
- about 75% of the time it will do that, the rest of the time, especially when starting in a slightly colder weather, it will jump to 2000rpm and just sputter our within a second or two. Starting again and holding the revs at 2000 by pushing on the accelerator and slowly letting go, it will remain running
as normal.
- The idle rpm at standstill will float between 50 or so rpms up and down again constantly but at random interval.
- At stable cruising speed, there is a similar floating from time to time
- Stepping on it has no stutter or hesitation, it just takes off like an ironball shot out of a cannon
- gears change smooth, and no power loss that i can detect

What i have done so far:
* MAF filter replaced
* MAF sensor in the airbox (the thingy with the wire and resistor) replaced
* All wires and contacts going to the MAF checked for breaks/damage and they are good
* All of the vacuum lines (thin ones) replaced, that looked even remotely worn
* The rest of the vacuum system hoses checked and they are good
* Temp sender on the engine replaced (Engine runs nice and cooled, no overheating issues what so ever)
* All the tubing / rubber from MAF box to the TB are solid and in great shape
* TB replaced
* Butterfly valve in the TB moves without any issues or dirt/dust resistance
* Brand new battery installed
* Generator works fine

Checking with the diagnostics computer, absolutely ZERO codes on the engine come up. There is a faulty ABS sensor on the FL wheel and the oil level sensor is bad, but the idle and startup issue was there way before these two issues came up.
The only thing that seems to work for a bit, is if i take the battery off for about 30+ minutes, the startup and idle issue seems to disappear, but only for a couple of days.

If anyone has any ideas, what else i can test or try, before throwing more money at her, please help. I love my gentle beast, but this anomaly is just driving me up the wall at this point.

Here's her (pic for street cred )

jfrahm

Fuel pressure regulator would be on my list to check. Get it to act up good, then shut it off and quick whip out a few spark plugs and see if they are wet. Could maybe be an injector or two, stuck and hosing.

BrianC72gt

It sounds like unmetered air. It makes up a larger proportion of all the air at idle, but very little at open throttle which is why it behaves badly at closed and nearly closed throttle. How's your PCV valve? torn diaphram? Stuck open from carbon buildup? Brake Vacuum Booster and hose? Inspect the air snorkel from the Air Flow Meter in the airbox to the throttle body at the manifold for cracks/leaks. Hole in the accordion elbow? loose clamp?

Move things around a little at idle and see if the engine rpms change. Push and pull on air hoses . Take an unlit torch and move it slowly around the intake manifold where it bolts to the engine, and around every seam and joint in the air inlet if the RPMs jump up from sucking in the propane or MAPP gas, then you've found your leak.

Are you able to monitor sensor output in real time? Ross Tech VAG COM? I'd check IAT, & Fuel Trim. If your fuel trim numbers are huge, then it could be trying to compensate for an air leak. Obviously this is better to monitor first when everything is running perfectly so you have a baseline number to compare it against.

AlexA8V8

Jfram, Brian, thanks for the input.

I believe you might be on to something. I'll get to that in a sec.
Did some digging around, checked all the CAI piping again, squeezing and everything, it's all in good shape and didn't seem to affect the idle. Now as far as the PCV valve goes, not sure - i mean it looks good on the surface but how would i test to see if that might be the issue, or at least part of the issue?
After a lot of rummaging around i noticed something i hadn't before. Namely one of the "bones" connecting the vacuum pump or pluf or whatever it is, was missing. This one below:





Because i couldn't find a replacement over the weekend, i fashioned a replacement out of some zipties and a sawed-to-measure shell of a pen (which works like a charm btw). After that, the engine idles much more stable as well as stays stable while cruising, but it still starts with the same "almost about to die" attitude, albeit, fewer jumps before leveling out to a normal idle. In addition, the engine and exhaust sound way more meaner and like a V8 should sound..
Whatever the leaver does, that the vacuum thingy pulls open, made the car louder.

Now what you were talking about, i've now noticed, how the idle, while much better now, has an erratic barely noticeable shake and occasionally i can smell gasoline while driving. So something's up with fuel going to the engine or not being used all up or i don't know.
But i feel like i'm getting closer to the cause.