2.5 TDI (AKN): Black Smoke, Power Loss - Help!

Hello everyone,

I hope for expert assistance.

My A6 with the 2.5 TDI (AKN, 195,000 km) is blowing huge black smoke clouds out the back when accelerating. If you only lightly press the gas pedal, it's manageable, but if I press it harder, a black wall forms behind me. At the same time, a air noise occurs that sounds like turning the ventilation on full blast. The performance is slightly weaker in city traffic, but especially on the highway, the performance is significantly reduced. However, I can't drive any faster than 110 km/h because the other road users wouldn't be able to see anything anymore; theoretically, it could still reach 150/160 km/h or so.

I checked the engine compartment and noticed that the hose connecting the mass airflow sensor (MAF) to the turbocharger had come off due to material fatigue. However, to my surprise, nothing changed after I installed a new hose.

So, I drove to the Bosch service center. The mechanic said that the engine is getting too little air somewhere, but first, everything needs to be disassembled to see where the problem is. An appointment wasn't available for 2 weeks. Then, he detached the small hose that is connected to the round container located directly above the turbocharger. Suddenly, there was no more black smoke! Did he disable the EGR valve with that? Or is that a different container that just looks similar?

As a test, I drove a few kilometers with the hose disconnected (although without closing the hose or the can). There was absolutely no black smoke anymore, but significantly less power. It's absolutely impossible to go above 110 km/h! The air noise is also no longer present during acceleration, but sometimes it appears very briefly when coasting without pressing the gas. However, I get the impression that the turbo is simply disabled without the hose.

What's broken? Or, what can be ruled out? } Because the Bosch service doesn't have any available appointments, I want to go to ATU, but I want to give them specific instructions right away, otherwise you'll just get ripped off again. I hope you have some tips for me. Thank you very much for your answers.

First, have the error memory read out.
I experienced the same problems with the AFB (which is very similar to the AKN).
1. LMM was defective.
"The boost pressure sensor in the left intercooler was faulty (it would intermittently spin for about 3000 km when I bought the car, then it completely failed)."
3. Due to the complete failure described in section 2, the hose connecting to the connecting pipe has detached from the right lower linkage (LLK).

Solution approaches:
For issues 1 and 2, definitely go to a and have the mass airflow meter (LMM) and the boost pressure sensor checked (possibly find someone with VAG-COM/VCDS and have them do a logging test).
3. Check the charge air cooler, especially the hose connections that are held in place with hose clamps.
4. If possible, remove the valve cover and inspect the camshafts (keyword: camshaft wear on V6 TDI engines), especially if you haven't already replaced them.

Best regards,
Nviduum.
alias
Bene.

New camshafts have already been installed.

Can't we narrow down the source of the problem by examining the fact that the air noise and the excessive soot disappear when the hose described earlier is removed?

Hmmm, I would say that the noise, the black smoke, and the lack of power are likely due to the intercooler system. As I mentioned, visually inspect all the hose connections and read the error codes.

Umm... it was still quite early this morning...

Okay, as mentioned above, the system might be leaking. First, check the VTG (variable turbine geometry) for proper operation, and then test the shaft of the turbocharger for play. It's possible that the turbocharger is failing.

I went to another workshop today.

The mechanic saw that a large amount of black smoke was coming directly from the turbocharger in the engine compartment.

My question:

It seems the turbocharger is damaged. Or, could a functional turbocharger produce that much black smoke under certain circumstances? After all, the turbocharger is still running, even though it's not performing at full capacity.

Just want to make sure that the turbocharger replacement is really necessary... it's quite expensive after all.

Thank you all!

The turbo itself shouldn't produce much soot... if it's producing anything at all. If it's emitting oil and the amount is so high that you can see it behind the car, it will be blue, not black (and usually it's too late because the turbocharger has already disintegrated).
It seems your engine isn't getting enough air. If the VTG (Variable Turbine Geometry) is deactivated and the hose isn't closed, does the problem disappear?
Log files from the air mass sensor, boost pressure sensor, and EGR valve would be very helpful for diagnosis.
Use the search function to look for information about TDI engines and soot. Gradually work through the results you find.

As I said, it could be the turbo, but it could also be something else entirely.

Excessive smoke usually indicates a faulty charge air hose. The noise also fits with this diagnosis.
When the technician disconnects the vacuum hose, the turbocharger no longer builds up pressure, which prevents any further leakage. Without pressure, the engine will naturally lose power.

I agree with my predecessor.
"Especially check the connections at the bottom of the lower control arms if you don't have an underbody protection."