A3 2.0 TDI: Strange Performance in Gears 1-3

Hello everyone,

A friend of mine had the cylinder head replaced on his 2.0 PD BKD engine because it was consuming excessive amounts of coolant.
After driving the vehicle for the first few kilometers, the turbocharger failed.
It has been replaced in the meantime.
Currently, the vehicle is exhibiting the following issue.
When accelerating in first gear, pushing the pedal all the way down ("pedal to the metal"), nothing happens until around 3000 RPM. Specifically, according to the VCDS diagnosis, the VTG (Variable Turbine Geometry) is operating very slowly, and the target pressure is not reached until later.
When accelerating, if you briefly release the accelerator pedal (let's say to 50% throttle) and then immediately return it to 100%, the car pulls through smoothly and powerfully as usual.
Das Problem besteht im 1. bis 3. Okay.

Here are the things I have already looked at.
- Exhaust systems are free (test drive with the exhaust pipe removed due to suspicion of a faulty catalytic converter).
- AGR (AGR verschlossen weil die Vermutung nahe lag das AGR nicht sauber schließt). No success.
- Check vacuum lines for cracks (a vacuum connection diagram at the N75 would be helpful here because I suspect something might have been incorrectly connected after the head installation...).
- No abnormalities detected in the actuator diagnostics.
- MWB inspections (with no apparent incorrect values except for the actual vs. target boost pressure difference).
- Function of the VTG (Variable Turbine Geometry) checked (although it's a new turbo, we wanted to be on the safe side).

Occasionally, the vehicle displays the error message: "Boost pressure below regulation limit."

I'm slowly reaching the end of my rope.
Apparently, the BKD engines also tend to have problems with the throttle body... However, the engine doesn't shake excessively when it's turned off.

Does anyone have any ideas?

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Does anyone have any ideas?

First, create a log file (data recording) from measurement block 01, 10, and 11 in gear 3, within the RPM range of 1200 to just under 4000. Accelerate with full throttle. Then, post the log here.

Perhaps create a second log where the accelerator pedal is briefly pressed to observe how the LD (load demand) develops in that scenario.

Then you need to determine whether the TL (likely referring to a pneumatic component) is receiving the correct vacuum supply in order to diagnose whether the VTG (likely referring to a valve or other mechanism) is jammed or not properly actuated/supplied with vacuum.

It can only be reliably determined using VCDS "and" a vacuum gauge by comparing the actual vacuum pressure with the duty cycle of the N75 valve.

From which source does the new TL originate? Unfortunately, it is not uncommon for substandard parts to be sold and installed... we have unfortunately experienced this several times here.

What exactly does the abbreviation "TL" stand for?
Okay, I'll test that vacuum thing.
However, I only have a boost gauge available that I could potentially use to get a rough estimate of the vacuum level.
What should the vacuum pressure be at the VTG (I've read something about -0.8 bar there)?

To try and fix the issue, we also attempted to manually adjust the VTG during the journey using a large syringe, but I suspect that even with the large syringe, we were unable to generate the full vacuum pressure required.

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What exactly does the designation TL refer to?

Turbocharger.

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Okay, I will test that thing about the vacuum pressure. However, I only have a boost gauge available that I could potentially use to get a rough estimate of the vacuum level. What should the vacuum pressure be at the VTG (I've read something about -0.8 bar there)

?

With full activation, it roughly achieves the necessary movement of the VTG (Variable Transmission Gear).

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To try and fix it, we also attempted to manually adjust the VTG during the journey using a large syringe, but I suspect that I was unable to generate the full required vacuum pressure with it, despite the size of the syringe.

It should probably be enough... You can check while it's stationary to see if it's sufficient or not.

In a properly functioning VTG (Variable Transmission Gear), the shaft should be able to move back and forth by approximately 10 mm.

Okay, we tested that, and the VTG (Variable Turbine Geometry) is working... We tested it with the exhaust manifold removed because I wanted to make sure the catalytic converter wasn't clogged, since a significant amount of oil had leaked into the exhaust system due to the turbocharger damage.

While stationary, you could also clearly hear how the soundscape of the turbocharger changed.
Okay, so I'm ruling out any mechanical issues with the VTG (Variable Turbine Geometry) for now (like a jam, etc.), since it's a new turbine.
We also checked the ease of operation, and everything seems to be working correctly with the VTG (vehicle traction generator).

I need to figure out where I can get a vacuum pump.

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I need to see where I can get a vacuum pump.

What do you need a pump for? You need a measuring device to compare the duty cycle of the N75 valve with the actual vacuum pressure, in order to, for example, rule out a stuck N75 valve.

"First, you could generate a log file as described above. Perhaps the cause lies elsewhere, for example, in insufficient airflow."

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Compare the key press ratio on the N75 with the actual vacuum pressure.

More tips:
- A vacuum hose is collapsing somewhere in sections 1-3 (the engine moves more noticeably within its mounts when running in lower gears).
- A valve is occasionally malfunctioning.
- In the vacuum system, someone with limited motor skills has broken or disconnected a hose connection or the vacuum reservoir, resulting in intermittent loss of vacuum depending on the position and condition of the hoses.
- The engine control unit is limiting the fuel injection and turbo boost due to another error, which is causing this unusual driving behavior. Are the cables/connectors to the mass airflow sensor (MAF) okay? Check the air mass values with VCDS, as well as other potential "limiting factors" such as the various temperature sensors (charge air, exhaust).

Best regards, Rainer.

Thank you for the tips!
I will also check everything.
Can you give me some tips on specific log files that I can show you to help assess the error?
Maybe I'm so focused on the details that I can't see the bigger picture anymore.

Regarding the LMM (lambda-by-mass) values, I must say that they looked quite good when I checked them during the drive, and they also appeared to be in a good range when viewed while the vehicle was stationary.

Logging data blocks 4, 8, and 11.

Check if measurement block 7 and 10 are plausible.

Okay, I tried to create some meaningful log files.
I hope they can understand the problem from this.

It's a shame that I misspelled it; I meant MWB 3 instead of 4. But it doesn't matter, the mass airflow value in one log looks plausible in relation to the boost pressure.

The turbocharger is not building any boost pressure, but the control signals from the engine control unit (ECU) are correct.

Please check in the basic settings mode, block 10 or 11 (engine idling), whether the variable geometry turbocharger (VTG) is moving at all. A movement of 1 cm of the adjustment rod is normal.

Is this charger new (an original part), or is it just another piece of cheap, poorly made junk?

"Baseline settings have already been checked, and the adjustment is correct. Strangely enough, it also runs well for a short time when you briefly let off the throttle. However, in the log from 3rd gear, it didn't run correctly even when releasing the throttle. The turbocharger was rebuilt by a company in Stuttgart with completely new components. Only the compressor housing on the exhaust side and the air intake manifold are original."
The old turbine had a shaft failure.

Okay... maybe I should sometimes step into the "minefield," always doing the same thing.

Based on my experience, I estimate the probability that something is not working correctly with the loader to be 80%.

That's exactly why being stingy isn't cool; it's actually quite stupid when you end up spending days searching for faults in your car.

"Man, that part is available as a new replacement from Garrett at affordable prices through parts retailers."

Please upload a log (MWB 8 and 11, 4th gear) where the car accelerates normally at full throttle. I'll predict the result in advance: The boost pressure will be high enough randomly that the fuel injection quantity limitation due to air mass won't engage.

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Based on my life experience, I estimate the probability that something is not working correctly with the charger to be 80%.

That is most likely the case.

However, before claiming a faulty temperature sensor (TL), it should first be checked whether there is sufficient vacuum pressure at the VTG sensor in case of a fault. Unfortunately, the duty cycle alone does not provide information about this.

Hello and thank you!
I will provide the log file again as soon as possible after work.
In my opinion, there should be sufficient vacuum pressure, but I don't have anything to measure it with and can only observe the control signals in the pit and judge based on that.
Engine on = fully engages the VTG (Variable Turbine Geometry).
If I bend the vacuum line, it immediately returns to its original position by more than 1 cm.
Once the hose is back to its normal position, the VTG engages immediately.

Regarding the company that repaired the faulty turbocharger:
They use new parts to repair the turbos.
Of course, a warranty is also included.

But I think we'll see more this afternoon when I upload the MWB 8 & 11 in 4th gear.

Thank you very much for the great support.

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If I bend the vacuum line, it immediately returns to its original position by more than 1 cm.

Connect the vacuum hose directly to the control unit? If you bend the hose, nothing should change at the VTG lever, because all the air that is sucked out of the control unit through this hose must return through the same hose.

If you bend the hose and the retractable VTG lever retracts as a result, then the control unit is defective (the internal rubber membrane may be torn or the unit itself may be leaking).