A8 surges slightly in 5th around 40-50mph
 

So here's the story, I provide pre-purchase consulting services and search out hard to find vehicles for local clients.

Well...

After much deliberation, my client decides on a 99 A8 Quattro with 99k on the clock. She drove it, I drove it (and I mean DROVE it) and the suspension is solid, runs like a champ, shifts flawlessly, interior and exterior are in great shape. I check the compression, no leaking fluids, nothing serious at all. The car needs a fan clutch as the current one is becoming a little noisy and a cover for the headlight washers on one side. She buys it and gets a 4yr unlimited mileage warranty as well (smart lady).

She drives it home and it's fine, the next time she gets in it she calls me up and say the trans is acting up. So I scour the forums, contact the dealer and read up on issues with this particular trans and find that it tends to be very problematic, generally in the form of a "thump" when downshifting caused by the torque converter locking up and letting go at improper times.

The issue seems to be that in 5th gear while maintaining speed, just cruising or up a gentle slope the engine revs up about 250rpms then drops back down and does it again and will continue until you either give it some gas or let up on the pedal. She thinks it's the trans and is dreading what the dealer will say (she made an appointment just for an inspection, I provide labor).

The dealer says she needs a trans at the cost of about $8000. Considering that the issue is a rhythmic rise and fall of rpms I suggest it's probably much less serious and likely a sensor. Definitely not a vacuum leak as it only occurs within a certain rpm range, other that this it runs and drives great.

So she brings her car over and I take it for a ride trying to duplicate the problem which I easily did in both 4th and 5th gear under light load in auto and tiptronic modes. I pull the intake system and inspect the MAF sensor, pull it, spray some cleaner on it, take her for a ride and the surge is much less apparent, this time only varying about 50-100rpm. I park the car, let her know it could just be a faulty MAF and I'll take another look when the torrential downpour going on outside stops.

She gets in it an hour or so later, takes it on some errands and calls me back and says that it's wonderful, no surging.

I'm looking for some input from you guys, I'm very familiar with BMW, Saab, Jag and Volvo but I've only owned one Audi in the past (00 A4 2.8Q).

But for all you A8 owners (or any fuel injected vehicle) that don't do this regularly, CLEAN YOUR MAF SENSOR!!! Don't let the dealer tell you your very expensive trans is toast if you're having this issue! I've read of numerous instances where this issues was simply fixed with a MAF replacement, I've heard of them dying as early as 54k, but more often around 80k!

Anyone who can provide me with any more insight on this problem would be doing me and the proud new owner of a gorgeous A8 a favor in the event that this issue comes back, if it does I'll replace the MAF sensor...

ALSO, make sure that if you are experiencing the dreaded trans "thump" you pull the trans pan and filter and clean the metallic sediment from the magnets in the pan AND check for build up around the valve body, pump, and solenoids. A simply cleaning may save you thousands of dollars, people who abide by audi's "lifetime fluid" claim are doomed, I recommend a flush and filter change every 3 yr or 36k, my Volvo 960 is approaching 500k miles and has the original engine and trans.

Well, i can tell you are a pretty sharp guy so i won't beat around the bush.
Its not a common complaint to have a slight surging problem with these cars, so that was unique. The dealer always replaces transmissions on the D2, so they probably just shotguned it and claimed it as a bad trans. Poor dealership technician diagnosis.
These cars (just like many other cars) use the MAF sensor as an imput for how much air is being sent to the engine to determine shift requirements. If the sensor is dirty/faulty it can indeed cause this problem. I would say you are on the correct path with changing the MAF sensor and indeed you should do so.
However, if she ever developes problems with the reverse gear....she's up shits creek. There is nothing you can do short of replacement or re-build, which most shops won't even attempt on an A8.
I totally agree with you on the fluid changes, maybe not as early as 36k (cough chevy) but every 50-60k would be best. The leading cause of transmission failures that i have found at the dealer is leaking fluid. When the fluid starts to leak, the clutches will slip and burn up. This also causes hard shifting, people ignore the problem until its too late. I always recommend checking the trans pan and torque converter seal area after every oil change. If there is a leak, get it fixed immediately and the trans should in theory, last forever.

Thanks for the quick reply, I don't really know the 4.2 V8 or D2 chassis that well yet, although after hours of inspecting hoses and diagnosing this issue I'm learning :D. I got a new job yesterday as well so I'm currently searching out an S8 for myself, I'm pretty jealous of the A8 I see in my parking lot every day.

So the problem returned and I found it to be a cracked vac hose at the front of the block (no clue how I missed this the first time...) and I also found a split in the longest hose with 4 or 5 bends in it coming from the throttle body (not sure exactly what it's for but I know it shouldn't be leaking). You know, the $55 dollar one from the dealer... So basically we have a very confused computer trying to compensate for the unmetered airflow, hooray no new trans!

So I ordered the hose, the dealer of course didn't have it and claimed that "we've never had anyone need one" which if complete crap as the one that's on the car is being held on with one time use clips that the parts guy said are not an audi part, I just think he's wrong... I also cleaned the MAF sensor again, this time really over-doing it and using about half the can of cleaner.

Long story short I temporarily patch the leaking hose with RTV sealant and replace the cracked vac line and we have a very happy A8 owner who just took the car on a 6 hour trip with no more surging and MUCH better gas milage and performance. That and she doesn't want to tear my balls off for paying me to find and inspect the vehicle then this issue popping up as soon as she gets it home...

Glad to hear it, why the dealer "says" they never give those hoses out is because most D2 A8 owners are pretty smart so they take the cars to a euro ace indy mechanic that won't rape them in labor. The dealer i used to work at charges $135 and hour and doesn't pay us for diagnosis unless the customer pre pays, so its basically a no win situation. One of the reasons i left. Definately tell the customer to keep an eye out for leaking fluid under the car, because if she starts to see a light brown fluid under the car, she needs to take it in immediately.

Thanks!
 

It's not leaking (yet...) but she actually lives in the same townhouse complex as I do so I'll probably see it before she does but I'll let her know to keep an eye out. I took the car for a ride today after she returned from her trip just to make sure it was acting as it should and it performs flawlessly, I just wish I would have caught the vacuum leaks the first time I had the engine covers off...

I'm going to be changing the oil soon as well so I'll be sure to check the trans while it's up on ramps.

Thanks for the right up. Mine had a slight surge in D at a stop. Could have been all the time but that is when I would notice it. Cleaning the MAF took care of it.:)

Glad I could help! Be sure to take a look at the vac lines near the throttle body and the front of the block as well (just to be safe). If you're idling high or rough in addition to surging that's likely the problem, or maybe an intake gasket leak. The car that had this issue had pretty brittle vacuum lines and it's still below 100k, almost all of them at the front of the block were splitting where they were coupled.

Sorry, I'm a maintenance nazi...

On the downside you may need a MAF sensor in the near future, not sure exactly what'll happen when it dies, but my Volvo wouldn't idle and ran about as well as a paraplegic when the MAF sensor failed.

Never hurts to have a replacement on hand (or in the trunk) so she doesn't leave you stranded.

Nah, these cars will run just fine with the MAF sensors disconnected, it just takes the ECM a few hours to adapt to not using it. I wouldn't recommend keeping a $400 MAF sensor inside a car where it could get dirty and flop around in the trunk or something.

In the box, in plastic with proper packaging of course and secured :D

When I had my Saab 900 Se Turbo I always kept a spare DI cassette with me because when that dies you're going nowhere...

but disregard that if it doesn't need the MAF to run, you can always drive shamefully slow