Hello all,

My A6 4F2 with 4.2 V8 (BAT) and 220'000km (136701 Miles) has some annoying problems:

- Rough idle and at low RPM. The engine shakes pretty bad and car has less power. At higher RPM it runs fine. So it happens when the car starts or goes to stop ( Red lights, ...) Sometimes the call will run fine for a few days. But on a bad day it will happen a lot. If I unplug the MAF, it fixes the problem. I have already replaced the MAF, but it changes nothing. With MAF unpluged the car runs fine can't reproduce the problem.

My mechanic says it might be some bad fuel injectors, some water in the fuel, or a bad head gasket but he said he did not find any oil or over-pressure in the coolant tank. He tried to swap spark plugs positions to see if it could be this, but apparently no.

I will try to put some isopropyl alcohol in my tank to see if it goes any better.

VCDS Faults as 11 september 2023
I guess the two last are because I unplugged the MAF sensor.

On 12 june 2023 I got these:

Any ideas, how can I try to find the cause ?

Other things my car has, but I think it shouldn't be related:

• The engine has some mini oil leaks in the middle front. But I don't need to refill too often
• The AC/Ventilation sometimes blows very hot air but only on left driver side and a bit on right passenger side, but the center blows cold air. This only happens after driving for a while and after sudden deceleration. Any ideas what this could be ?
  

 

The fuel trims should be expressed as a % +/-. I do not know the BAT engine on the 3.2 FSI you will get a figure for each cylinder. I am not certain where to begin. My first step would be to get the engine nice and hot and then use VCDS to measure the fuel system pressure. On the 3.2 fsi you do this at the high pressure pump. Your engine may not have a high pressure pump but the idea is the same. You use the Advanced Measuring Blocks feature and graphing to see the fuel pressure over time. On my car over 10 minutes or so it will rise to double the original figure and stay there. Leaking injectors won't behave this way. Leaking injector seals show up as normal - that that is were the fuel trim values begin to help.

Candidly, use the fuel trim data to learn whether the sensors are asking for more fuel (+) or taking fuel away (-). Use this to inform a systematic check for air leaks or fuel system issues.

Just my 2 cents

 

Vag-com checks with engine running would help by logging MAF etc. With the MAF disconnected, the ECU is programmed to run on assumed stoichiometric 14.7:1 air fuel ratio. This allows the car to be driven and not leave you stranded simply due to a MAF failure. Without MAF the engine will be a little shy of max fuel economy and max power but it will run. Assuming your existing MAF and replacement MAF are correct and functional part and not contaminated then the initial suspect would be faulty MAF wiring to ECU. Inspect the MAF connector pins and make sure they are not stretched or corroded, look for cracked wire jackets, etc. Start by vag logging / graphing the MAF along with engine RPMs during a drive.

Ignore the rich and lean codes for now as they are likely byproducts of the source problem.
The codes to resolve are:

16485 - Mass Air Flow Sensor (G70)
P0101 - 008 - Implausible Signal - Intermittent - MIL ON
16486 - Mass Air Flow Sensor (G70)
P0102 - 002 - Signal too Low - Intermittent
16497 - Intake Air Temp. Sensor (G42)
P0113 - 002 - Signal too High - Intermittent


Why add iso-alcohol to the fuel tank?


Need a vag scan of Address 08: Auto HVAC system as it will report which sensors and or dash flaps that are not working properly.

 

My mechanic said it could help if there is water in the fuel tank.

Last days the car ran fine. Today I logged some sensors using the vag-com (Air intake temp, Mass air flow, throttle position, Engine RPM). But as there were no noticeable hiccups today, I guess I will try to do it again when the car is acting strange.




There is no errors in this module. I will try to log some values when the problem occurs again.

 

Okay so I just came home from work, and I was logging trim values. The car drove fine but when coming to a stop in front of my garage I let it idle a few seconds and then it started having the rough idle thing. So here you can see at the end of the graph some values goes way up.

Graph of trim values. You can see at the end the car wag rough idling.

So then I logged MAF values to check:

You can see how the engine RPM is unstable, lots of little waves on the line. And MAF values are a mess. In the middle, I put some throttle, we can see it fixes MAF values.

Then I stopped the car and started it back on and the rough idling was gone:

Here we see RPM is more stable and also MAF values. I put a bit of throttle in the middle.

So this is very strange because I replaced the MAF because the old one was behaving the same and no changes. Maybe I have to say that in the past I already changed the MAF like 2 times I think. Maybe with 1-2 year interval each. Symptoms were the same, but after changing it, it fixed the problems. But not this time...

I checked the connection the the MAF but it doesn't look very bad to me ??
I found this in readiness check if this can help ?

Maybe my VCDS is very old ?

Then I checked the new faults code (I removed again the cable to the MAF before checking the codes):

 

If your VAG-COM software is old, just download the newest version.

 

You Bank 2 Long term fuel trims indicate the O2 sensor is telling the ECU to take away fuel. Can you take the MAF off and test it electrically? If it passes, I would be tempted to look for fuel injector leaking on Bank 2. I would start by getting the engine nice and hot connecting VCDS and doing the equivalent of this test (this is for direct injection) for your car you are looking for fuel rail pressure under advanced measuring blocks. Then watch/graph it for ten minutes after shut down - it should rise with the heat soak and then after ten minutes or so start to fall.

 

Ignore the readiness for now.

None of the data shows the air intake sensor acting up which suggests it was working during the point-in-time data logging events. Would be interesting to see what it reports when it is not reporting correctly to trigger the DTC.

The LTFT could be going overly negative because of the bad MAF readings and suggests running rich. This is why your mechanic suggested possible “injectors” especially if they are leaking extra fuel. Normal readings are ideally between +5 -5. Could try a high quality DI fuel injector cleaner additive to the fresh tank of Tier 1 highest available octane fuel but caution do not exceed the instructions concentration.

Interesting to note the MAF readings are all over the map only at idle and appears to be working above idle. You may need to inspect the MAF wires all the way back to the ECU for cracks. Could try reseating the ECU connector with battery disconnected. With wiring diagrams could perform continuity measurement from ECU connector to MAF connector 5 wires. Measured with a volt meter, what is the battery voltage at idle when MAF is acting up?

Although your MAF connector pic is pretty good, it is a little hard to see the pins. Look close for damaged or spread pins in each of the 5 terminals. Could do same visual pin inspection at the ECU connector for MAF wires. The red circled one in this pic shows the clearest view of how the two sides are spring loaded and should be in contact with each other when not connected.
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I am thinking AudiAllTheWay has forgotten more than I know....

 

😅 We are saying similar things.

If engine is indeed running rich then a functional BxS1 will tell ECU to pull fuel. If available then testing MAF in another working 4.2 is a good idea as well. Watching heat bloom in the fuel rail after hot engine shutdown is a good test for stuck open injector too. DI engines should not have raw fuel in cylinders after hot shutdown. An undetected stuck open injector will eventually destroy the engine.

It is possible there is more than 1 issue such as stuck injector(s) and a problematic MAF connection and possibly intake air temp sensor.