AFB Issues: Insufficient Air Mass, Fluctuating Boost?

Hello.

After having the mass airflow sensor (MAF sensor) replaced by the mechanic, we discovered that the MAF sensor readings were still incorrect (759mg/stroke). The mechanic's response was, "That's just how it is with the V6 engines; they don't read correctly anymore... but the engine is running well."

"Since I didn't like that statement, I 'borrowed' a 'new' mass airflow sensor (MAF) from a Bosch service center and tested it with VAG-Com, and the values were identical. However, I logged the boost pressure and noticed that the values from the MAF sensor were actually better in the boost pressure overshoots (see attachment)."

Does anyone have an explanation for this problem with insufficient air mass and excessive pressure fluctuations?

For testing purposes, I deactivated the EGR valve (by disconnecting the hose and inserting a screw). This resulted in higher MAF sensor readings at idle, but the readings remain the same at full load. So, shouldn't that indicate that the EGR valve is okay? (see attachment)

I tested the VTG myself using suction from my mouth on the hose, and it moves smoothly and completely from bottom to top. However, it reaches 0-50% faster than it does from 75-100%. Is this still okay?

The engine has 130,000 kilometers on it, and the air filter is new.

I'll probably take a look at the camshafts sometime this weekend.

Thank you for your suggestions.

Serge.

mserge wrote:

Does anyone among you have an explanation for this problem with insufficient air mass and excessive pressure fluctuations?

What value do you consider to be the "correct" air mass, and where does that number come from?

As long as the soot limit is not active when the LD (lean NOx trap) is engaged, and the air intake path to the engine is sealed and the EGR system is functioning correctly, I wouldn't be too concerned about "low" MAF (mass airflow) sensor readings.

Hello Ulf,

"I'm taking the 'correct' value from the repair manual, which states that it should be above 750 mg/stroke at 3000 RPM in 3rd gear with full throttle. The diagnostic tool (model 1551 or similar) at my mechanic's shop showed a target value of approximately 862 mg/stroke! This value is often exceeded by other AFB engines (>800 mg/stroke, as seen in this forum). Furthermore, I believe that the engine is not delivering its full power, which I will have tested in a performance test."

What worries me even more is the fluctuating boost pressure. This fluctuation (which you can clearly see in the diagram) is much too severe (dropping from 2.2 bar to 1.8 bar, then rising again to 2.1 bar, etc.) and disrupts the smooth running of the engine. It needs a range of 2000-3000 RPM for the boost to be regulated, and then you shift up a gear, and the whole process starts again.

Thank you for your comments.

mserge wrote:

What worries me more effectively is the fluctuating boost pressure. This fluctuation (which you can see very well in the diagram) is much too severe (dropping from 2.2 bar to 1.8, then rising again to 2.1, etc.) and ruins the engine's smooth operation (it needs a range of 2000-3000 RPM until the boost is regulated, and then you shift up a gear, and the whole process starts again).

Have you read it yet?
I'm sorry, but I cannot access external websites or specific files online. Therefore, I am unable to translate the text from the provided URL.
I'm sorry, but I cannot access external websites or specific files online. Therefore, I am unable to translate the text from the provided URL.
Please log the limitations and also check for more unusual causes of insufficient airflow.

Hello.

Yes, I've gone through all of these texts, and I'm currently checking them.
I will also log the boundaries, as you recommended.

Thank you very much .

Hello Ulf,

You were right about the low mass airflow (MAF) values; the control unit is setting the limit, not the MAF sensor (see the log file), although it's a narrow margin, it's still the case.

However, are these fluctuations in LDL levels considered "normal"?

I curiously checked the diesel temperature (52°C at an outside temperature of 30°C) and the intake air temperature (43°C at an outside temperature of 30°C). Could these also lead to a loss of power?

Greetings.

Hello,

In my opinion, the problem lies in the boost pressure control. You almost never reach the maximum boost pressure of 2300 mbar in the log. If you have less boost pressure than specified, the amount of air being drawn in is naturally also lower.
I would suggest not focusing on the LMM (likely referring to a mass airflow meter) for now, until you have repaired the boost pressure control system.

Hello.

"Who says I *have* to have 2300 mbar of boost pressure? The engine control unit outputs a target value, and the boost pressure control system achieves that target value (with two overshoots and undershoots). For me, this is either something to be considered 'NORMAL,' as the friendly mechanic always says,' or the problem lies with the engine control unit, which isn't demanding enough boost pressure..."

Hi,
Regarding the target values:
http://www.tuneline.at/news/images/audi_v6tdi/2/bild6.jpg

Hello.

Can someone please tell me if it will be helpful to clear the learning values of the engine control unit, or if a reset of the engine control unit will make a difference?

How do I perform a reset like this? Is it enough to disconnect the battery overnight, or do I need to unplug the control module (MSG) or something?

The problem is that the MSG isn't releasing enough fuel injection volume... see the last log.

I'm getting my performance tested this afternoon, so I'll know exactly what's missing.

Serge.

It won't help because no learning values are being saved.

Hello.

Yesterday, I took a look at the NWs (presumably referring to some mechanical components), and well, the rest you'll probably know... you can definitely tell they've been used. They seem to be quite worn (about 3/10 mm, I'd guess). Both the cams and the back of the NWs show clear signs of wear.

Can someone tell me if I should replace it immediately, or if I should keep using it until it gets even worse?

As mentioned at the beginning, I have insufficient air mass and fluctuating boost pressure, which results in a corresponding power curve. While it indicates 156 horsepower, it's achieved in a somewhat uneven manner. It feels a bit stiff in the lower RPM range (with the air conditioning on), but very responsive in the higher RPM range. Otherwise, there are no other negative signs.

"Does anyone have a parts list of the components needed to replace the NW (needle valve)? I already have the NW numbers (from this forum), but I still need the pushrods, hydraulic components, and seals, basically everything you need for a repair."

Thank you very much.

Okay, the camshafts have now been replaced, but the long-term fuel trim values are still present, and the mass airflow sensor value is still low, although not critically so.
The engine is now running more smoothly, but it's still not great. What really bothers me are these boost pressure fluctuations under full load. I have a power curve from when the car had 30,000 km and one from when it had 130,000 km. I'll scan them and put them here. In the diagram, you can compare the boost pressure curve with the power curve!

Will it be beneficial if I make the VTG rod slightly longer to prevent the boost pressure from increasing too rapidly?

Will it be beneficial to temporarily install a turbocharger from an AKE engine with 180 horsepower? Will an AKE turbocharger fit on an AFB engine without any modifications?

Thank you very much.

Serge.

"I'm experiencing similar issues with my vehicle. I noticed a small indication when I performed a diagnostic test using a diode instead of the mass airflow sensor (MAF). At that time, all the values were back within the normal range. Ulf suggested that a leak in the intake area could also be the cause. This would explain the successful diode test and the malfunctioning behavior despite the new, functioning MAF sensor. The MAF sensor might be reading lower values because it's detecting unmetered air being drawn in. Does your car produce excessive smoke? If not, that's an additional indication of a leak, because there's usually enough air present to burn the fuel cleanly without producing smoke. However, the engine control unit (ECU) might think there isn't enough air, so it reduces the fuel injection amount (to the amount specified in the soot map) and also lowers the boost pressure."
Why did you think of the camshaft? To what extent do the values of the mass airflow sensor and the boost pressure influence the system?

@Zak1976

Hello, so I have an AFB engine, which is a V6. I don't think it can function with a mass airflow sensor (MAF) replacement using diodes. I also don't believe there's a problem with excessive air being drawn in, as the distance between the MAF sensor and the turbocharger is relatively short, and everything seems to be okay in that area.
Regarding the camshafts, you can check the knowledge databases to see that these engines have quite a few problems with them. If the camshafts have worn down by more than 0.5mm, the valve timing changes, and the engine receives less "air" because the valves no longer open fully and don't open at the correct times. Since mine are now new, I can rule out this factor, as well as the mass airflow sensor (MAF), which was replaced at 130,000 km. However, the strange thing is that the MAF sensor is never the limiting factor, so the loss of power must be due to something else.
I'm going to adjust the VTG (Variable Turbine Geometry) to a slightly less aggressive setting and then observe how the boost pressure behaves. If the boost pressure is within the acceptable range, then the air mass and power output should also be fine.

Here are the performance curves for vehicles with 45,000 km and 130,000 km on the odometer.
At 45,000 km, the mileage seems acceptable to me, but not at 130,000 km due to the pronounced "bump" in the graph.