nathanwind

Ok, so I've got a 2006 A8L, 4.2L V8, 85K miles. Out of the blue I've been dealing with the below CEL codes from VCDS. Cleared it, and came back within 5-miles just about every time (went through this 3 or 4 times). So, yesterday I pulled the airbox out, checked all hoses/connections to make sure everything was tight/snug and no airleaks - found nothing. Reassembled, but - like before - code(s) came back immediately after a few miles. Also (for kicks) pulled the O2 sensor harness plugs (the ones to the left of the airbox) to check the connectors and make sure all was well - no signs of any issues.

I pulled the airbox to assess since I was *hoping* my issue may be similar to one from another member as described here:
https://www.audiworld.com/forums/a8-...r-w12-2861994/
...but alas, no luck.

Am I missing anything else "obvious" to check for? I was sort of overly optimistic that it may have been just a leak from the airbox causing it to run lean since as mentioned above that seemed to be an issue for someone else - but seems no such luck. Is it odd for both B1 and B2 to be going off simultaneously? And within, literally, just a few miles? I always thought it took a few more cycles for the system to come back to "readiness" status. Or is that sort of standard? I'm fairly mechanically inclined, so don't mind digging in a little further....but I'll admit this one has got me stumped. I hate to admit defeat and bring it to the shop, so any input is welcomed to assist troubleshooting.


[-------------------------------------------------------------------------------
Address 01: Engine Labels: None
Part No SW: 4E0 910 560 P HW: 4E0 907 560
Component: 4.2L V8/5V G 0010
Revision: --H01--- Serial number: AUX3Z0E9776339
Coding: 0007773
Shop #: WSC 02313 785 00200

2 Faults Found
18627 - Oxygen (Lambda) Sensor B1 S1: Signal too Low (Lean)
P2195 - 008 - Implausible Signal - MIL ON
Freeze Frame:
Fault Status: 11101000
Fault Priority: 0
Fault Frequency: 1
Mileage: 137163 km
Time Indication: 0

Freeze Frame:
RPM: 2558 /min
Load: 92.9 %
Speed: 38.0 km/h
Temperature: 82.0°C
Temperature: 15.0°C
Absolute Pres.: 0.0 mbar
Voltage: 14.224 V

18629 - Oxygen (Lambda) Sensor B2 S1: Signal too Low (Lean)
P2197 - 008 - Implausible Signal
Freeze Frame:
Fault Status: 01101000
Fault Priority: 0
Fault Frequency: 1
Mileage: 137169 km
Time Indication: 0

Freeze Frame:
RPM: 1540 /min
Load: 19.6 %
Speed: 39.0 km/h
Temperature: 86.0°C
Temperature: 33.0°C
Absolute Pres.: 0.0 mbar
Voltage: 14.097 V

Readiness: 0000 1101

-------------------------------------------------------------------------------

mishar

It doesn't look like vacuum leak but checking all vacuum hoses won't hurt. In the mean time I would suggest injectors cleaning. Techron or similar. It would be faster to take injectors out and clean them on a bench. Also cleaning MAF sensor.

MP4.2+6.0

You have the codes on both sides of the motor, so causes me to think it is not the O2 sensors or something electrical connected to them. Still possible, but seems like less likely.

Codes are pre cat (S1's) so that is also confirming it isn't the cats. By exclusion you come down to air and fuel. You have checked air it sounds like, but nonetheless that'a probably where I would still go. You also seem to describe/imply it came on suddenly, causing me to think something broke or came open. Also keep in mind though folks/dealers occasionally find some really hard issue like a bad manifold (that was an S8/V10 IIRC).

I would start at MAF airflow wise and go from there. The MAF of course could be screwed up enough it just isn't reading right and closed loop the system can't overcome the error. I would expect other codes too, but keep MAF of the suspect list. If any K&N ever touched it, more so. Meantime, at least try some MAF solvent cleaner.

After MAF, look at anything else headed to manifold; sounds like you have--large hoses, clamps, any signs of deterioration, throttle body bolt up, any signs of cracking, etc. Look at the vacuum hose hook ups, suck valve connection near the main intake plumbing, etc. Once you enter the manifold, since codes are on both sides, I'm less prone to think it is something there (injector seals, something cracked or open, etc.).

Could be fuel side too but I have little experience there since all mine have been reliable since back to mid 80's. I would think there would be other symptoms or codes though. Again, since both sides I would not be thinking it is right at the engine/injectors but something further back. Throwing a fuel filter at it is easy, but seems like shot in dark and not well coupled to sudden onset of codes. Nets to scrubbing air side a lot more and continuing to suspect an MAF failure as another.

nathanwind

I just cleaned the MAF (with CRC MAF cleaner), letting it dry then I'll reinstall, clear the codes, and take it for a spin.

I'll recheck all the plumbing again while I'm waiting just to be "safe".

If it pops back up, and the MAF is a main suspect as mentioned above, *how* does one go about actually testing the MAF for proper functionality? Or should I just pony up the $100 or so for a genuine or Bosch MAF to completely take that out of the equation as a culprit?

nathanwind

Test drive completed, CEL came back on....pulled the codes, same 2 as originally listed. May go the injector cleaning route tomorrow as Mishar recommended (certainly can't hurt). Going to do a little more research as well, and depending what I find may try a new MAF to see if that cures my woes.

nathanwind

I did find this thread with a similar CEL code/issue:
What is WRONG with my car?? - Texas Audi Group Forums

Granted the thread was not circled back on/closed out, but the last post alludes to a vacuum leak (as both of you suggested), but also the "valve on crank case that allows engine to regulate amount of fuel in and out". What is this valve that is being referred to, do you guys know? Sounds like it may be something worth checking/replacing.

mishar

That valve is pressure regulator and yes, it can cause problem like this. It would be more pronounced on a high engine load. So floor it and see if it stars misfiring and stuttering.

MP4.2+6.0

The pressure regulator idea is logical too. I haven't dealt with pressure stuff on FI since my 1985 C3. Back when it was CIS (non-electroinic), you could tweak the mixture by playing with some tiny shims that were against a spring that controlled the pressure.

MAF; you can check it by running VAG COM in metered (recorded) mode. One of the measuring blocks is for calculated MAF flow in grams/second. The rule of thumb is take 80% of rated HP and that is what you should see for MAF flow right up near the shift point (combined if it has two like S8 or W12). Thus my old C5 was rated at 300HP, so I was looking for 240 g/s. And in fact when I tested it suspecting an MAF issue--one of my many tranny shift related items on that one--I found around 210 g/s. Post change IIRC I found 250+ g/s. As in, it was under-reading air flow. There I didn't have your codes if I remember in general, but it deal feel down on power some pre-change. The closed loop element w/ O2 sensors can correct for this within reason, but if the O2's are forcing the change there is also a lag factor as it hunts around some to constantly decide correction factors. Open loop (cold) it won't really know what to do beyond MAF reading and its then current adaptation. And yes, at some point MAF's are cheap enough now it can just make sense to change them out. BTW, "Rebuilt" if it is quality often means new sensor in a maybe prior used tube, so it's essentially new as well.

aTOMic

Definitive way to check FPR is to pull the vacuum line from it and look for gas-should be dry and very faint gasoline smell; failure is when diaphragm breaks allowing fuel through. Other check is to hook a Mity-Vac up to it and see if it holds vacuum.
These engines are such a Medusa's head of vacuum lines! Shops use fog machines but I'm thinking I may hook my shop=vac with its 25-foot hose to the intake and listen closely (hey,it worked to diagnose a sunroof drain)!

Also, don't the O2 sensors share a ground or something? Could be a bad connection at the mutual connection.... unlikely but I know you gurus know.

mishar

No ground connection there, but they share power connection to the heaters. It comes from the fuse 6 in the right dashboard fuse box. I don't know if luck of power there would trigger a faulty code or cause lean condition.