JWeb

Recently, my '01 A6 2.7T 6-sp with APR tuning and 106K mi has been demonstrating some severe stumbling upon acceleration under load. The car starts, idles, and drives under light loads beautifully. There is no CEL. The gas mileage is acceptable (~24.5 according to dash computer - notwithstanding the calibration errors now that I have a chip).

In any gear, under heavy load, the car stumbles. It feels like a shudder with a frequency akin to popcorn popping in the microwave (just to give you an idea of the rate of stumbling).

I immediately checked the TBB and DVs. The TBB looks fine. I removed each DV and did the press-diaphram / hold finger over vacuum port and they held fine. (They are 710N valves, replaced about 15K miles ago).

I then performed a leak test using the poor-man's tester. Aside from the slight hiss that I believe is normal leak down, I could not detect any leaks at 10psi. I was afraid to go higher.

I then pulled out my trusty VAG-COM, logging blocks 115, 003, and 031 during 2nd and 3rd gear pulls from off-idle to about 4250 RPM. The most relevant data came from the 3rd gear pull (posted here as a JPG image of the edited VAG log).

<img src="http://pictureposter.audiworld.com/57486/vaglog.jpg">

To me, the most interesting entry is time stamp 23.84 (I realize the log times of the Group B and Group C entries for that line are slightly later... let's ignore for now). Notice that "actual" boost has dropped off. This was during the "stumbling" mentioned previously. But, I also note that my MAF reading went from 190 g/s down to 139. Also, timing is significantly retarded at this point.

My questions are:

1. What came first, the chicken or the egg? That is, did boost pressure drop because the MAF reading was off and the engine wasn't receiving the proper amount of fuel/air and power (exhaust flow) dropped off? Or, do I indeed have a boost leak and the loss in power decreased the amount of air flow through the engine and hence the MAF reading? If so, am I doing my leak test wrong as I didn't find any leaks?

2. Can I diagnose this further by unplugging the MAF sensor? I've seen that suggested in the archives, but I'm not sure of the specific consequences.

3. Is the timing information relevant?

4. Do I need more measurements as conclusions can't be made from one line of VAG data? (I don't like driving the car hard with this problem, thus my hesitance to wind it up in 3rd gear for an extended period of time).

I also checked measuring block 032 for my LTFT:

-0.8 -3.1 -0.9 -4.7

They certainly seem normal.

Finally, I scanned for codes. Nada. At one time, I was getting:

17927 - Camshaft Adjustment; Bank 1: Malfunction
P1519 - 35-10 - - - Intermittent

which I thought was relevant to the problem. But, I cleared that code and it hasn't been back.

Any ideas from the brain-trust? I hope I've provided enough info. What else can I test for with my VAG?

Thanks in advance...

S4_Cinci (I S'edTFA)

and this points directly to the bad MAF. Someone just posted about their B5 S4 dying due to bad MAF - unpluging it kept the car running enough to get it to the shop for repair. Might be worth a try, as I think it won't hurt anything to unplug the MAF, but I am not 100% certain so hopefully someone else can verify that such a trial won't do more harm than good.

Edd W

partially collapsing under high flow cause it to restrict the inlet? - This would explain almost everything you have here I think.

At the time of the problem, the engine is still getting enough fuel but tyhe air supply is suddenly restricted, so the ignition advances as far as possible to try to give as long burn time as it can.

It could also of course be an air leak which for some reason was not picked up in your test. Of course, you could test up to a bit over the 2000 mb - this equates to a bit over 28psi so you were perhaps a bit too conservative when testing? - I am not sure it is safe to do a static test that high, but someone else here will undoubtedly give further advice.

Just out of interest, is block 31 primary oxygen sensors, if so, it shows everything on the ignition side is fine as it kept within tolerances.

Edd W

I am not happy these results point to a bad maf because the fault would be more linear, not sudden.

JWeb

... it's easy enough to check. Is this something that is common?

As for static pressure up to 28psi, I think that could be too high. I do remember reading somewhere that things get ugly around 15psi. Also, I don't think my pressure fitting (rubber PVC coupler) will stay on much beyond 10psi. Anyway, I'd be interested in what others think.

JWeb

... and wouldn't read good/bad/good/bad/good/bad?

It seems to me if the electronics are beginning to fail, this could be the symptom.

Any idea if unplugging is safe and recommended for a short test? If so, can I drive 10 - 15 miles in open loop (MAF unplugged?)

JWeb

... with APR tuning, wouldn't I expect AT LEAST 250 g/s and probably more like 280 g/s?

IanB

Which is like saying, "I have been reading a **** load of posts on AW lately about chips and performance" the drop in bost is usually because the ECU makes it so. From what I can remember, you hit a peak in fuel and the ECU slams on the breaks and lowers boost dramatically to keep everything going, but at the same time manipulates timming to cover for the potential of unburnt fuel.

People have reffered to this as the car 'Falling on it's face" basicly I have no idea what is happening for sure, but from the log and what you have posted, things seem to be that way. Although, the requested boost throws me off. Because if my deductions are correct it should be requesting a lot less boost, and getting it.

So just my .02.

S4_Cinci (I S'edTFA)

OK by me - just throwing out something that might be a quick check to verify MAF function.

JWeb

... after warming up, I got into the boost and the problem persists. Thus, I think I can rule out the MAF. (The CEL did come on, which I am assuming is because of the unplugged MAF but I haven't checked with the VAG, yet).

I don't want to rule out the DVs. Is the finger-plug method accurate enough to test the viability of the DVs? Would a failing DV cause these symptoms, anyway?