MAF Blues '98 2.8 24v AHA (225K miles)

Problem:
New MAF now the car idles fine won't die but hesitates off of idle to ~1800RPM, then runs perfect.
History:
A few months ago I started to experience some idle problems with my 2.8. This got progressively worse over the next month or so with the only code being thrown was a P0102 (16486) Mas Air Flow sensor (G70) : Signal too low intermittent. Sometimes, the car did not want to idle at all (when warmed up) other times it ran fine. I tried cleaning the MAF which bought me a little more time but soon I was back to the same old thing. One day in frustration I disconnected the MAF and presto ..... the car was running like it should except for a very small stumble just off of idle. Being this was my busy season and the wife didn't have any more problems with driving the car, I delayed forking out the big $ for a new Hitachi unit (078 133 471c) till I had more time to work on it. I picked up the new MAF, decided to disconnect the battery for a half day before installing it just to start at the base line. It ran fine while the engine was cold but as soon as it warmed up it started to have minor idling problems. This wasn't too bad and slowly over the next week the car has returned to a perfect idle. The only problem is that as the idle got better, a stumble off of idle got worse. Right now the car runs perfect at idle and at 1800+RPM, but give it too much gas trying to get between the two, and it starts to stumble (this could be described as a 'miss' or a 'cough' or maybe even like a wet ignition problem). In the last two weeks of driving I have yet to get a code.
Where do I go from here? I have access to VagCom but really don't know what to look for.

Thanks in advance for any info.

 

My first guess would be to look at short and long term fuel trims on each cylinder bank. If there is a difference between the banks you might have a vacuum leak somewhere. Also check the o2 sensors. The pre-cat (air/fuel) o2 sensor should switch back and forth between lean and rich every few seconds (0.2v - 0.8v I believe). If not, the sensor may be going bad. Coolant sensors can also cause engines to do funny things when there are going bad, check to see if it is reading an acceptable temperature and not 1000 degrees or some temperature that is way out of range.

This is just basic trouble shooting advice. I work as an auto tech at a small shop but am by no means a guru.

 

Thanks for the reply, I can run the numbers just not sure what I am looking for.
Water temp shows a constant 185 F.
Here's a sample of the readings of group 002, 032 & 033.
this is 2.8 5-Speed
Note: it won't stumble if you rev it up higher between gears, I am purposely shifting low to force the stumble.
Starting from a stop at time stamp 116.7
Shift to second at 121.26 it stumbles between 122.02 & 122.78 then runs clean.
As always, runs perfect at idle and over 1800.
It looks to me like bank 2 short term % goes up to 12.5 & 13.3% just as I get the stumble, then back to normal.

If my assumptions above are correct, what type of air or vacuum leak
would happen just off idle (under a load) then work perfect up the RPM range?
>>>>>>>>>>>>>>>
EDIT: I think I have this backwards, my short term Lambda are -12.5 & -13.3% (Negative) which means that I am running rich at the stumble not lean? ????? any info would be helpful.

I am going to try and attach the Log sheet to this note. Hope that works.

 

Is the water temp always at 185 F even if the car is cold? There are usually 2 coolant temperature sensors, one that goes to the instrument cluster and one the feeds data to the ECU. The ECU uses the engine temperature in its calculations for air fuel mixture. Not sure if this is your problem though but it could be. I personally have never seen a coolant temp sensor fix anyones problems but I have heard stories from other mechanics.

Long term fuel trims stay steady whereas short term fuel trims usually jump around +/- 10%.

It seems you have a lot of negative fuel trim values meaning your engine is running rich. And it is happening on both banks. Does your exhaust smell rich?

Is that data from driving around and then shifting from 1st to 2nd gear?

 

(1) Water temp acts normal, cold with a cold engine then up to ~185 and doesn't vary much from that point. Changed the temp sensor a few years back it seems to be working correctly.

(2) From what I understand, short term should fluctuate between +2 or +3 to -2 or -3 in a constant throttle situation. Mine seems to be all over the place. I took it out on the flat freeway, held a constant 3000 RPM for miles and it would act normal about 90% of the time but then there would be periods of 10 to 20 seconds with only positive numbers between +5 & +10.

(3) Yes that was the other day, today I am getting a lot of positive numbers. No rich smell but unlike my Golf or Chevy PU (whose tail pipe is squeaky clean) the A4 is dark and you do get some black on your finger if you rub inside.

(4) Yes, got the car up to temp, came to a stop sign (you can see it idle at 720) the took off.