Mass Air Flow Sensor Issues: A6 2.5 TDI Diagnosis

Hello.

I'm having a problem with my A6 and could use some suggestions.

A6 V6 2.5 TDI Avant, Engine Code: AFB, Tiptronic transmission, Year of manufacture: 06/1998.

'There's a noticeable lack of power, I estimate about 10%, which is especially evident in the top speed. On a good day, it used to reach 230 km/h according to the speedometer, but now it only reaches 215 km/h.'

Do you have VAG-COM?

I then realized that the expected air mass value should be 879, but 765 was too low. So, I tried a new mass airflow sensor at the VAG workshop because it was still under warranty. Unfortunately, it didn't work; the values from the new sensor were even worse than mine, around 730 instead of 879.
'The boost pressure is okay at a maximum of 2100. I've done various test drives, for example, once without the air filter, which should measurably increase the airflow to the mass airflow sensor (MAF), but there was no change; it showed the same value as with the air filter installed. Then the idea came up that air might be bypassing the MAF somewhere. I found and fixed a leak in the intake hose, but there was still no change in the values.'
At VAG, the idea was that it could be the catalytic converter, as the car had consumed approximately 1.5 liters of oil over 750 km due to a defect (crankcase ventilation), which weakened the overall engine performance. A new catalytic converter was installed without success.
Then they had the idea that the problem might be related to the exhaust gas recirculation system, but even with a properly functioning EGR valve, there was no change.
When a new mass airflow sensor (MAF) and fuel injectors were installed about 3 months ago because the car wasn't reaching speeds of over 200 km/h, the MAF was showing values that were either at the target level or even 10-20 units above it.


Has anyone ever experienced a similar problem?

- The timing belt was replaced 15,000 km ago after being misaligned by one tooth on the crankshaft.
- Approximately 10,000 km ago, all the fuel injectors, the mass airflow sensor (MAF), and the crankcase ventilation valve were replaced.
- At 60,000 km, the valve lifters were replaced due to wear.

Greetings, Ferengi.

Wow, I'm really glad I didn't get a V6 TDI.
What we have in the workshop for repairs due to poor performance and excessive oil leaks is really not a pretty sight.

Your problem might be caused by a faulty mass airflow sensor (MAF).
When I was still working at BMW, we once had a record of three brand new mass airflow sensors failing one after another.
It means installed, test drive, then immediately removed. So, even new mass airflow sensors can be defective.

No, I don't think so. We tried two new ones, two days apart, from different deliveries. Both were faulty. And then there was one from a completely different vehicle, but its condition was very poor. So, I no longer believe that the problem is with the mass airflow sensor (MAF).

Greetings, Ferengi...

HM,

How was it?
You installed the mass airflow sensor (MAF) three months ago, and it was working fine then.

Quote:

When a new mass airflow sensor (MAF) and new fuel injectors were installed about 3 months ago because the car wasn't reaching speeds of over 200 km/h, the MAF was showing target values or even 10-20 units above them.

Is he not okay now?

Quote:

I then realized that the expected air mass value should be 879, but 765 was too low. So, I tried a new mass airflow sensor at the VAG workshop because it was still under warranty. Unfortunately, it didn't work; the values from the new sensor were even worse than mine, around 730 instead of 879.

While it's unlikely that the LMM failed within 3 months, what if it shows poor readings?
Okay, it's important that all leaks can be ruled out after the LMM (likely referring to a specific process or measurement).
Secondly, it is important to compare the target and actual values of the mass airflow sensor only where the boost pressure also corresponds to the target value.
Otherwise, I would double-check the voltage supply to the mass flow meter (MFM). If the supply voltage is too low, the output signal will also be correspondingly lower, assuming the MFM itself is functioning correctly.
Otherwise, try replacing the mass airflow sensor (MAF) with a diode and check if the values are correct then. If that's the case, it would definitely be the MAF sensor.

Hi,
How about the valves and NW (needle valve)? Were they installed due to a supercharger defect, or are they typical for wear on V6 engines?

Okay, so the mass airflow sensor (MAF) was replaced three months ago and it was working correctly then, with a value slightly above the target setting. For example, the target value is 879, but it was reading 890.

During the timing belt replacement at the beginning of the year, the compression was also checked. Everything was okay. Regarding wear on the turbocharger: both cylinder heads were replaced about 50,000 km ago due to turbocharger issues...

Regarding diodes... does anyone know the pinout of the mass airflow sensor (MAF)?
Here, in these tips, you only ever find information related to the 4-cylinder TDI mass airflow sensor, which is different.
The V6 version has a temperature sensor that is visibly located on the outside; it looks like a diode and probably has a different pin configuration. I have the complete wiring diagram for the engine, but it only shows one positive wire and all the other wires go to the control unit. However, it doesn't specify what signals those wires carry.

'When I get tired of it, I'll build myself an adapter that allows me to simply increase the value and that's it... but that should be the last resort.'

I own an Audi A4 2.5 V6 TDI, manufactured in 1998, with engine code AFB, and I'm experiencing very similar problems.

The LMM (lambda measurement module) showed a maximum value of 710 instead of 890 (measured at 3000 RPM / 3rd gear), indicating a power loss of approximately 30 horsepower. No errors were stored.
The car only reached a speed of 210 km/h.
Two new mass airflow sensors were tested at the Audi workshop, but without success.
Then tests were performed on the: exhaust gas treatment system, turbocharger, and fuel injection system (7 days). Everything was within tolerance. I heard someone say, 'There's nothing we can do here.'


Heute im Bosch-Service wurde Motor-Steuergerät probeweise getauscht. Success?
LMM 900!


Greetings.

Rafael.

Hello Rafael,

'That sounds like a glimmer of hope. I was actually planning to disconnect the engine ventilation system at the intake manifold again and see if the low mass airflow sensor (MAF) value might be related to air being drawn in through the ventilation system, bypassing the MAF. However, that's contradicted by the fact that the oil consumption is normal... I had a similar issue before where a valve in the crankcase ventilation system wasn't closing properly, which prevents oil from being sucked out of the engine under high load (i.e., strong vacuum in the intake manifold). In that case, the oil consumption was immense... but everything seems to be fine now.'

Okay, I would now like to build a circuit that increases the mass airflow (MAF) sensor reading.
'There's a voltage between 0 and 5V coming out of it, depending on the mass flow rate. And if you could simply increase that by 0.2 to 0.3 V, you might be able to correct something. But that would have been extra work.'

'I haven't done anything about it for the past few months. I basically accepted the lower performance because I wasn't willing to spend €2,000-€3,000 on troubleshooting and replacement parts to get those missing 20-30 horsepower.'

Now, the crucial question:
What is the cost of a new engine control unit (ECU), and where can I find it at the lowest price?

Greetings, Hans-Georg.

What is the cost of a new engine control unit (ECU), and where can I find it at the lowest price?

Bye.

I decided to go with a used control unit. It costs around 300 euros. In addition, there's the cost of coding it at the Audi service center, which is another 30 euros.

Greetings.
Rafael.

As I mentioned before, I need to replace the engine control unit (part number 8D0 907 401 A) in my car.
Sometimes, the only available control unit is part number 4B0 907 401 H (removed from an Audi A6).
Does anyone know if these two control units can be installed interchangeably?

Bye.
Rafael.