Need your advice regarding a T4 with an AUF engine, the EGR valve is malfunctioning...

Hello.

I would appreciate your advice. It's about a T4 with a 2.5 TDI engine, model code AUF, and an automatic transmission. The current mileage is 104,000 km.
The vehicle is a modified version, adapted for people with disabilities, that belongs to a friend.

The problem is excessive black smoke during acceleration and occasional rough idling. My initial suspicion was the EGR valve, so I wanted to rule it out first. I put a piece of metal to block it, and that's where things got curious. The engine ran terribly with the EGR blocked, and it seemed to stumble during idle. I removed the metal plate, and it went back to being relatively normal, but then suddenly it started behaving the same way as when the EGR was blocked. However, it could be fixed with a slight tap of the accelerator pedal. Then, I logged the parameter block 3 during idle, and I saw that the EGR valve was closing itself whenever the idle became rough. It can be seen in the log between lines 293 and 294. Starting from line 536, it can be observed that after tapping the accelerator pedal, everything returns to normal.

My questions are as follows:
1. Why does the engine run poorly (like a bag of nuts) with a closed AGR valve, or even with an AGR valve that has been adjusted using VCDS?
2. Why does the EGR valve close automatically when the engine is idling? Could it be that the N18 has a defect?

I would be happy if you could understand this and share your thoughts.
No, seriously, I'm happy if you can help me with your knowledge.

Best, Tobi.

Quote:

2. Why does the EGR system close itself during idle? Could it be that the N18 has a problem?

That's perfectly normal; the AGR valve is time-limited in many TDIs when idling.
The log for MWB 3 is also unremarkable.

It's quite curious that it runs unevenly with the EGR valve closed, but runs better with it open.
I can only imagine something like an extremely early start to fuel injection, which the EGR system somewhat mitigates due to the slower combustion.

I would like to see MWB 2, 4, and 13 at idle .

Quote:

so, metal sheet in between, and right there is where something curious comes in for me. The engine was running poorly with the EGR valve closed, like a bag of nuts, and it was always slightly misfiring at idle.

Did you, while working on it, perhaps unintentionally scratch or scrape at any deposits in the EGR valve? This can also happen if these deposits make their way into the engine.

Hi Tobi,

Could you check if the intake passages are clear and sealed?
Based on how the car is behaving, it seems like it might be getting more air when the AGR (Abgas Rückführung) system is open.
received when it is closed.
Excessive black smoke may indicate that the engine is receiving more air than is necessary for complete combustion.
Available upon request.

Best regards, Micha.

Thank you for your responses.
@Malte1408
Unfortunately, I won't be able to create any more logs for about a week. I will do so as soon as possible. I did observe MWB 13 while it was idling, but I didn't manage to log it. Everything seemed normal, and as far as I can remember, the values were never more than 0.9 apart.
The mechanic might have messed something up when changing the timing belt, right? I'll ask him when and where he had it done. Here's some information I can gather before I see him again, and we can log it.

@Weasel
Regarding the intake system, I thoroughly checked everything from the turbocharger back, as we had also removed the intercooler to install the bracket.

Thank you.

Tobi.

Is the car performing as expected? Did the smoke and/or the rough idling appear suddenly, or was it a more gradual decline?

Hi Daniel,

Unfortunately, he can no longer really answer many of these questions because he had to continue using the vehicle, and over time, he simply had to accept its condition.
I suspect it either developed gradually or is related to the last timing belt replacement.

@ all

Please let me know what other types of logs you might need so that when I'm there next time, I can collect all the necessary logs for a proper assessment of the situation.
Malte had already mentioned MWB numbers 2, 4, and 13. I will definitely do the same next time.

Best regards,

I should definitely check the air filter box and see if it's damp; it's possible it got some water in it, which could also explain the AGR issue.

Is the idle rough immediately after a cold start, or when the engine has reached its operating temperature?
After a cold start, I would suspect the glow plug system.

Otherwise, as mentioned before, the initial spraying process itself, not the issue that occurred with the timing belt.

I will create more logs as soon as possible, and then we can proceed.

Thank you very much.

Best regards,