Hey guys,

I just picked up a 2000 a4 1.8t from a dealer auction. Overall the car runs and drives great, but of course like any used car, it's not without it's problems.
I've had a ton of cars, but this is my first Audi. I mostly do all my own work, and have no problem getting my hands dirty.

I have searched the forums but I'm still a bit unsure of what the issue could be.

First Problem:

The car threw a check engine light. Pulled the code (with a generic OBDII code scanner.

P1128 - Bank 1 mixture adaptation - System too lean

The car has no misfires or other problems, and drives right except for a slightly erratic idle.

What the car does, is when it's in gear (tiptronic) sitting at a traffic light, the idle seems to rhythmically surge up about 50 or so rpm, from about 800 to 850 rpm. Almost if someone is tapping the gas ever so slightly and letting go. It doesn't drop at all below 800 rpm.

If you let off the brake, but have the throttle still closed, the car feels as though that's exactly what your doing, crawling forward with ever so slight lunges.

When the car is in park, the idle still flutters a bit based on the tach, but it's much less and not as easy to "feel" in the car.

From my research here, and other places, it seems that P1128 is generally associated with one of two sensors failing, first the Mass Air Flow Sensor (appears to be most probable due to the fact that the idle is erratic), 2nd choice is the precat 02 sensor.

Any ideas or way to narrow it down to one of the two? Anything else I should focus on or try? Resetting the code led to a momentary wildly fluctuating idle, which settled down, but did the same thing.. The code reappeared within 75 or so highway miles.

Thanks in advance,

George

 

The motor is "hunting", as (you are correct) either the MAF or an O2 sensor is failing. I had the same problem on my 1998.5, and decided to replace all of them (so not sure how to narrow it down to 1 of the 3). Don't buy the MAF new, as they are expensive. I ran a rebuilt MAF that cost me $70 for 5 years with no problem. Also just bought both 02 sensors, and the problem went away.

 

since the idle doesnt fluctuate by very much and there is no drivability problems really, i would say you have a leak somewhere over a bad MAF or o2sensor. check your vac lines.

 

i have the same problem and a very experienced audi mechanic that is actually from germany worked on my car a few times to find the problem and its a bad rear O2 sensor. i am positive that its an o2 sensor for you to because you explained exactly what my car does.. i just havent fixed it yet cuz my emissions arent due till april.. also try turning off the ac while its doing it and see if that makes it stop.. then turn it back on and see if it starts again.. lol thats wat mine does

 

the rear O2 sensor has no input on engine performance. it is there for emissions purposes only, so if it were failing all you would get is a check engine light and a code. the car would not even otherwise know it wasnt working. the primary O2 sensor is the one that has an input on fuel mixture so that one does matter if its not working.

when you turn the AC on you are putting a load on the engine. that makes it slightly harder to rev up and down like was happening and will smooth the idle out. you may also have a small leak somewhere, or the primary O2 sensor is just starting to go causing an only slightly bad reading and only slightly bad idle. out of curiosity what is your MPG like?

 

this has been goin on since i had the car but it seems like i get good mpg... the indicator in the middle of the dash doesnt work if you wanted to know wat that says..im 98% sure he said rear o2 sensor but ill call him today to ask which one he said was bad.. the car when driving performs fine, just when im stopped for something and the ac is on while it bounce and blow out hot air.. also i noticed that the engine temp sometimes will randomly fall back to the cold side while im driving then slowly work its way back to normal.. i bought a new coolant temp sensor to install but i cant get passed all of the electrical wires to install it so im gonna wait til i do the o2 sensors and just add that in there..

 

Well here is the Vag scan:

He seems to think it was the MAF, saying that the reason the MAF is giving inconsistent readings, he didn't find a vaccum leak, He said something about the 02 sensor being forced into these readings by a bad maf... More explanation at the end of the scan....

Saturday,15,August,2009,11:04:12:59860
VCDS Version: Release 805.4
Data version: 20090602



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


Chassis Type: 8D - Audi A4 B5
Scan: 01 02 03 08 15 16 17 25 35 37 45 55 56 57 67 75 76 77

-------------------------------------------------------------------------------
Address 01: Engine Labels: 4B0-906-018.lbl
Part No: 4B0 906 018 Q
Component: 1.8L R4/5VT G 0006
Coding: 07251
Shop #: WSC 06335

3 Faults Found:
17536 - Fuel Trim; Bank 1 (Mult): System too Lean
P1128 - 35-00 - -
18014 - Rough Road/Engine Torque Signal from ABS: Electrical Malfunction
P1606 - 35-10 - - - Intermittent
17521 - Oxygen (Lambda) Sensor; B1 S1: Internal Resistance too High
P1113 - 35-10 - - - Intermittent
Readiness: 0010 1001

-------------------------------------------------------------------------------
Address 02: Auto Trans Labels: 8D0-927-156.lbl
Part No: 8D0 927 156 AP
Component: AG5 01V 1.8l5VT USA 3132
Coding: 00002
Shop #: WSC 00000

No fault code found.

-------------------------------------------------------------------------------
Address 03: ABS Brakes Labels: 8E0-614-111-EDS.lbl
Part No: 8E0 614 111 A
Component: ABS/EDS 5.3 QUATTRO D10
Shop #: BB 24334

1 Fault Found:
01203 - Electrical Connection between ABS and Instrument Cluster
03-10 - No Signal - Intermittent

-------------------------------------------------------------------------------
Address 08: Auto HVAC Labels: 8D0-820-043-1D.lbl
Part No: 8D0 820 043 Q
Component: A4 KLIMAAUTOMAT D56
Coding: 00140
Shop #: WSC 06335

1 Fault Found:
01206 - Signal for Duration of Ignition Off Time
27-10 - Implausible Signal - Intermittent

-------------------------------------------------------------------------------
Address 15: Airbags Labels: 8D0-959-655-AI8.lbl
Part No: 8D0 959 655 K
Component: Airbag Front+Seite 1002
Coding: 00104
Shop #: WSC 06335

No fault code found.

-------------------------------------------------------------------------------
Address 17: Instruments Labels: 8D0-920-xxx-17.lbl
Part No: 8D0 920 980 CX
Component: B5-KOMBIINSTR. VDO D13
Coding: 02244
Shop #: WSC 65535

No fault code found.

-------------------------------------------------------------------------------
Address 35: Centr. Locks Labels: 8D0-862-257.lbl
Part No: 8L0 862 257 N
Component: CV-Pump D11
Coding: 16140
Shop #: WSC 06335

7 Faults Found:
01367 - Central Locking Pump Run Time Exceeded (Likely Leak)
35-00 - -
01371 - Alarm triggered by Door Contact Switch; Driver's Side
35-00 - -
01372 - Alarm triggered by Door Contact Switch; Rear Passenger Side
35-00 - -
01370 - Alarm triggered by Interior Monitoring
35-00 - -
00991 - Interior Lights
28-00 - Short to Plus
00955 - Key 1
09-00 - Adaptation Limit Surpassed
00956 - Key 2
09-00 - Adaptation Limit Surpassed

-------------------------------------------------------------------------------
Address 45: Inter. Monitor Labels: 4B0-951-173.lbl
Part No: 4B0 951 173
Component: Innenraumueberw. D03
Coding: 00101
Shop #: WSC 06335

No fault code found.

End ---------------------------------------------------------------------

Oh one thing I forgot to add...

Engine block code 32, fuel trims...

At idle: -2.x% At Upper engine speed + 23.x%

So it's adding a LOT of fuel to try to compensate for what it thinks is unmetered air due to the maf, but it's pretty much ok at idle.

According to what I found on a different forum, this is explained by this:

You can check your current state of trim by using VAG-COM or equivalent to look in Group 032 in your engine measuring blocks. The first two fields will have percentages. The first field tells the fuel trim at idle (Additive). The second field tells the fuel trim at elevated engine speeds (Multiplicative). Negative values indicate that the engine is running too rich and oxygen sensor control is therefore making it more lean by reducing the amount of time that the injectors are open. Positive values indicate that the engine is running too lean and oxygen sensor control is therefore making it richer by increasing the amount of time that the injectors are open.

It is totally normal for both the first and second fields to be something other than zero. In fact, zeros indicate either you just cleared codes (which will reset fuel trim values) or something isn't working properly. If values get too far away from zero, it will cause a DTC (fault code) and can set off the MIL (commonly referred to as the Check Engine Light, or CEL).

Specifications for normal operation are usually somewhere near +/- 10%.
In general, an out-of-spec value in the first field (Additive) indicates a vacuum leak since it is mostly present at idle, when vacuum is highest. An out-of-spec value in the second field (Multiplicative) indicates a fault at higher RPM, and may point to a faulty MAF.

So I think I'm looking at a new MAF. Opinions? Ideas?

Oh and having to get a rebuilt ABS module (I knew that though).

Other than that the car for the most part seems fine...

George

 

Just wanted to post a follow up, mostly because I hate threads where people never post the real solution to the problem!

Replaced the MAF this afternoon with a brand new Bosch one from ECS. Then cleared the code. So far no codes have returned, although I only took it for about a 10 mile ride.

The idle seems to have smoothed out greatly.. It still pulses a couple times after to a stop but doesn't do it consistently like it would before at a stop light, and the lunging at idle (in gear with no foot on the brake) is gone. It seems as though the car is slowly "learning" the new maf, and adapting back to normal. As I was driving, the idle seemed to get smoother and the pulsing less pronounced. I guess the next couple days will be the ultimate test....

Thanks again for all the advice!

Also forgot... the car had a somewhat dirty K&N drop in filter in it....

I put it back in just because I didn't have a clean new filter to put into it, but do you think that may have been the cause of the MAF failure? Or was it just it's time to go at 111k?

Thanks Again,

George

 

I clean my maf filter with a q-tip and rubbing alcohol. Sometimes the oil of off the K and N gets on the sensor. I cleared the CEL and normally that fixes my problem.