VW AFN Power Loss: Diagnostics & Solutions

I have a VW Passat 1.9L TDI with 81 kW (110 hp), engine code AVG (AFN). I've been experiencing a noticeable loss of power recently and am trying to find the cause. Yesterday, after doing some research on forums, I did a test drive using VAG-COM. Unfortunately, I can't really make sense of the values in the Excel file. So, I'm asking the experts here: what can you read from the table? Things like boost pressure, lambda readings, etc. I would be very grateful for any help.
The file is located at:


The first file shows a VGA-Com MWB engine running at 3.8 and 11 (presumably referring to some measurement or setting), in the third gear, starting from 1300 RPM and reaching almost 4000 RPM with full throttle while climbing an incline.

http://www.compu-help.biz/picture/fahrt1_3gang.xls

The second file shows a VGA-Com MWB engine running in the fourth gear, starting at 1300 RPM and reaching almost over 4000 RPM with full throttle while climbing a hill.

http://www.compu-help.biz/picture/fahrt2_4gang.xls

I've noticed that the injection values are not matching. What could be the reason for this? Or are all the values within the normal range?

Sorry, I don't know your engine, so I have nothing to compare it to. If my engine were to provide such values (which it does ), I would rate it as follows:
Your engine's torque only starts to increase significantly around 2100 rpm, which I find quite late (but I don't have any comparison data for your specific vehicle). As a result, you experience relatively low airflow below 1800 rpm, which triggers the smoke limitation system at that speed.

neueinsteiger wrote:

"I've noticed that the injection values are not matching." What could be the reason for that? 3) Or are all the values within the normal range?

1) They are actually never the same because of different values. As far as I know (the description is just a "?"), the first one represents the driver's desired power output (your foot), the second is the torque limit (software, it can only deliver a certain amount of torque), and the third is the smoke limit (I get so much air, therefore I can only inject x mg/h). However, the third value shouldn't be the lowest.
"I'm experiencing this behavior due to a lack of vacuum pressure. I'm going to investigate the low-pressure regulation system."
"According to the data, it shouldn't lose power above 2000 rpm. However, mine exhibits the same problem at similar values. As I mentioned, in my case, it's due to a lack of vacuum."

3) According to the data, it shouldn't experience any power loss above 2000 rpm. However, mine exhibits the same problem at similar values. As mentioned, in my case, it's due to a lack of vacuum...

The air mass is located far down, at approximately 3000 RPM, around 880mg/r. That's a bit low. My suggestion: Check the airflow sensor according to the instructions/technical article, verschandeln it, and then perform another measurement test. }

Okay, I'm ready. Please provide the German text you want me to translate into English. Just paste it here, and I will give you the translation.

@mulle
Okay. However, the upper part of the combustion chamber isn't within the soot reduction zone. Could this cause a loss of power?

"I always start by checking the soot limit value. If there's a problem, I look at the lambda (air-fuel ratio). If the lambda is also incorrect in that area and the mass airflow sensor shows a significantly higher value than it should, I suspect the mass airflow sensor. This is because with a low lambda value, it will inevitably report less air intake. Is this the wrong approach? Perhaps the mass airflow sensor is more prone to errors in the opposite direction (reporting too little air mass, leading to insufficient turbo boost). Where do you start troubleshooting if you don't have any comparison logs from a known good engine (MKB)? For example, I know that the AKN engine tends to report relatively low air mass values."

The principle is always the same: the air mass is too low. Nevertheless, the boost pressure should be built up (possibly with a slight delay). However, the fuel injection amount is reduced due to the detected air mass in order to avoid soot. This happens with the AFN engine, and also with the V6 TDI. That's why it runs, but with reduced power. That's how I would explain it.

You're probably right. Just for my understanding: In this case, was the mass airflow sensor (MAF) also replaced initially, especially if it had been in place for a while? Generally speaking, the MAF is often a weak point.

In my case, I also replaced the mass airflow sensor (MAF), but unfortunately, it wasn't the MAF that was causing the problem.

mullemaus wrote:

The amount of fuel injected is reduced due to the detected air mass in order to prevent soot formation. 2) Therefore, it runs, but with reduced power. That's how I would explain it

.
"Good explanation, but if I could see that, I wouldn't be asking. Is the soot limit not in the last column of the quantity limits? It doesn't seem to be limiting at the upper end, even with decreasing air mass. I thought the mass airflow sensor (MAF) couldn't explain the power loss. Also, the value up to 880 seemed like an indication (significantly above the target), suggesting it can still measure high values. Mine never measures that far above the target (relatively speaking), even with overshooting and a functioning MAF."
As I understand it now, can the LMM reduce performance through a different mechanism than the soot cutoff (it reduces the amount of soot, but not necessarily by exceeding the soot limit)?
"From 2000 rpm onwards, the soot limit intervention doesn't actually engage (except for a single measurement point). I've always read here that as long as the soot limit intervention isn't active, the log won't show any indication of power loss, even if the air mass is potentially low. I thought a faulty MAF sensor only becomes critical when the soot limit intervention kicks in."

In my case, I had similar readings. At the time, I thought it was the mass airflow sensor (MAF). After replacing it three times, I discovered the problem was with the vacuum system.

The LMM should be above 1000. 880 is significantly too low. I only briefly glanced at the log, but I think I saw that the fuel injection amount was reduced (4 or 5mg?). The data in the third section of the log was also very strange? Weren't there fabricated data from 1991, etc.? I need to take a closer look, I think.

mullemaus wrote:

The mass flow rate should be above 1000. 880 is significantly too low.

Okay, aha . I'm missing the background information...

Quote:

"Sorry, I'm not familiar with your engine, so I don't have anything to compare it to."

@newbie
Could you please post the logs again for comparison once the LMM has been replaced? By the way, there's a replacement part available.

Off-topic.
Why can a 4-cylinder engine sometimes achieve gains of over 18%, while a V6 typically doesn't even reach 5% (based on my logs and those of other colleagues)? Furthermore, the soot reduction system in the 4-cylinder engine doesn't kick in as quickly, even with larger deviations downwards, whereas it activates much faster in the V6 (again, relatively speaking). Is this to force us to replace them more often?

@newbie
Could you please post the logs again for comparison once the LMM has been replaced? By the way, there is a replacement part available...


As far as I can see, the information is being provided sparingly. No, withheld. According to another forum, it has already been exchanged.

Herr Antje wrote:

OT Why can a 4-cylinder engine sometimes achieve gains of 18%, while a V6 typically achieves no more than 5% (based on my logs and those of other colleagues)? Furthermore, the soot reduction system in the 4-cylinder doesn't kick in even with larger deviations downwards, whereas it activates much faster in the V6 (again, relatively speaking). Is this to force us to replace them more often?

Hi Tom,

It might not be immediately obvious.

For example, if the opacity limit is reached at 1000 mg of air/stroke, and the corresponding diesel injection amounts are only 1 mg above the torque peak (e.g., 50 and 49 mg), then it looks very tight in the engine control unit (ECU) 8-log display, so that even the slightest weakness in the mass airflow meter (MAF) or a drop in fuel pressure could push the engine into opacity.

However, if the engine actually receives 1150 mg of air per stroke, the turbidity log values remain at 50 mg, even though there is still a 15% reserve in the air mass – but this reserve is NOT reflected as a correspondingly higher level of turbidity in the log (logically, one might expect something like: 50 mg + 15% excess = 65 mg of diesel as a turbidity limit).

The maximum permissible air mass in the MWB 3 is just as arbitrary: for the ASZ in G4, it states 850 mg, while for the Polo it's 1000 mg - but I don't know what significant differences these values should actually result in in practice.

So, the whole thing is partly a matter of programmer's discretion regarding the maximum values for Kf, which in real-world operation can sometimes be exceeded significantly (but unnoticed).

@newbie
It's not really ideal when we aren't given complete information about the background; otherwise, it just becomes a guessing game. Is there anything else you haven't told us (e.g., N75 replaced, vacuum pressure measured, catalyst bed already empty)?

"My AKN (air intake manifold) shows similar values. However, the air mass doesn't decrease as much at higher RPMs. In my case, it might be a vacuum issue, which I definitely need to check. Regarding your air mass problem, you could also try running a log without the catalytic converter/exhaust system. It's possible that yours is struggling against high back pressure."

@mulle
Then I was closer to the solution more due to luck than due to knowledge .

@ulf
That's interesting. I didn't know that. However, my injector has a slightly lower pressure (due to the lack of vacuum), which results in it spraying less fuel. When the engine is idling, my air mass is at or slightly below the target value.

Expressed in values read from here, the AFN should be 850, but under intact conditions, it delivers 1000. The soot limit intervention starts well below 800. Relatively speaking, my AKN (if it were intact) would have been around 830-850. It already operated within the soot limit range just above 800.

The values are slightly lower for the AKN engine because it has less displacement/stroke volume; therefore, these values are only provided as an example.

Quote:

According to another forum, it has already been exchanged.

Das ist dann schon das 3. Forum
http://www.passat3b.de/phpBB2/viewtopic.php?t=67596&highlight=

At least, he also withheld from them (as I quickly read) that a new LMM was installed .

I'm just here to read and I'm not willing to gather information myself.

It seems he lost track of things with the three forums and is no longer responding. At least he could have answered our tips and questions.