2.0 TDI CUNA - Constant Shaking at Steady Speed / Software Adjustment

Good day, everyone.

I am turning to you as a last resort. And here's the problem I'm having:

Briefly, here's some background:
I own a Skoda Octavia RS 5E, TDI version with the original 184 horsepower, model year 2016 (the 4x4 (DSG) with Euro 6 and NOx storage catalyst) with engine code CUNA.
Last summer, I decided to have a software optimization done by a tuner. A colleague with whom...
The same Octavia (model year 2015, Euro 5 standard, and without all-wheel drive) had previously chosen the same tuner and had had good experiences.
I received a file about it, and I was impressed by the performance, the torque, and the acceleration.
However, after about 3 weeks (at that time, the car had approximately 6000 km on the odometer), this issue of jerking or shaking at partial throttle/constant speed occurred for the first time. (A detailed description of the problem is provided below.)
Initially, it occurred rarely, then increasingly often – unfortunately, it could not be reproduced. Both with a cold engine and with a warm engine.
Subsequently, I had an in-depth discussion with my colleague, who, like me, uses the same software and has a virtually identical vehicle.
He has no problems whatsoever, so he's going back to the tuner. New file uploaded, but performance/torque is lower.
Unfortunately, the problem hadn't gone away. I then got used to it, drove it around for several months, and decided in January.
and lo and behold, the problem was gone. No more stuttering, no more sputtering – nothing.
I thought, "Okay, it was probably the file – I must have caught a bad tuner" (even though the colleague doesn't seem to have any problems), and I soon started missing the extra performance.
Last week, I decided to go with a different tuner for a custom engine tuning, hoping to get "something more personalized."
He made a very professional impression – the Octavia runs incredibly smoothly and has great acceleration – unfortunately, it's experiencing the same problem again.
Therefore, I now suspect a technical problem on my end, rather than a genuine software-related issue.


Now, let's describe the problem:
It's a constant, low-speed vibration with a clearly audible "whooshing/hissing" sound.
This occurs within a speed range of 40-70 km/h when driving in 3rd and 4th gear (between 1200 – 2100 rpm).
Maintaining a constant speed on a slightly uphill road (in the partial load range).
It occurs more frequently when driving manually (with a DSG transmission) – rarely or never with ACC (Adaptive Cruise Control).
It sounds and feels like an airflow in the intake/exhaust system is encountering excessive back pressure, causing the air column to break up or become turbulent.
Or, to put it simply, the turbocharger would be trying to force air in, but it wouldn't be meeting any back pressure, so the air would flow back and become turbulent.
As mentioned, unfortunately, this issue is not reproducible. However, I've noticed that it occurs much more frequently in the summer than at colder temperatures.
Initially, I suspected the AGR (or one of the two), but since it doesn't always occur, you could actually rule it out.
I once had a leak somewhere, or a membrane tear in some kind of valve (e.g.,...). I suspect it might be related to N75, but I haven't found anything concrete about it.
Interestingly, the problem only occurs when the system is "tuned," while my colleague doesn't experience the issue (even though his system is also optimized).
Therefore, I conclude that there is some issue related to the increased boost pressure, as no problems arise in the original condition.
Several loggings (showing parameters like boost pressure, EGR rate, torque, etc.) have also not revealed any anomalies.
In addition to everything else, I've noticed that when you suddenly "release the gas," the familiar turbo flutter occurs.
However, this wasn't present since the beginning of the software adaptation, but only started occurring from the point when the constant shaking at a steady speed also appeared. Similarly, this "turbo flutter" issue no longer occurred after the original software was reinstalled.
In addition to everything else, I have the feeling that it doesn't accelerate quite evenly. Sometimes (unfortunately, not always) it feels like it comes in waves.
Unfortunately, there are no error codes stored, and despite many discussions with "friendly" mechanics, I haven't been able to find a solution.

Do you have any other ideas about what might be causing this?
Thank you in advance for your help!

Best regards,
Timo.

Hi,

Can you check if the jerking or noises only occur during DPF regeneration, with low ambient air pressure (or when driving at a relatively high altitude), and between 1300-1800 rpm?

If so, it's likely that the programmed target boost pressure in that specific range is set a bit too high, causing the turbocharger to start pumping.

Since the effect depends on the mass airflow, a DPF with more ash inside will amplify this effect.

However, if you feel the effect even above 2000 rpm or while driving at a constant speed, it's not due to the issue described above.

Best regards, Rainer.
PS: You can also record the resulting sounds and upload them here using the "select file" button, etc.

Hi Rainer,

Thank you for the quick response.
The phenomenon doesn't only occur during regeneration. However, I also thought that it might only happen when the DPF is relatively full. I will read the data and check if it's better after a forced regeneration.
The speed range definitely matches your description.

Furthermore, after your input, I just realized that I've never encountered this problem in the mountains (I'm from Switzerland).

It does not occur in automatic mode either.

I will try to make a recording...

Quote:

I will read that and check if it's better after a forced regeneration.

Having a lot of Russian filter material might be beneficial, but be careful, as it won't necessarily reduce the amount of ash present.

You should read (and post) the ash mass/ash volume and also check the differential pressure in the DPF, along with comparison values.
Okay, I'm ready. Please provide the German text you want me to translate.
Okay, I'm ready. Please provide the German text you want me to translate.

Best regards, Rainer.

The difference between the EU6 engine and the EU5 is significant because the EU6 has two EGR systems: a low-pressure EGR system and the traditional high-pressure EGR system, which is also found in EU3, EU4, and EU5 vehicles.

Therefore, the two vehicles are very different. Additionally, it should be noted that there are very few (tuners) who truly understand what's involved, especially in the area of MQB diesel engines, and who can create their own files. The probability of getting different "approaches" from two different tuners is virtually zero. In my opinion, searching for a hardware defect in this case is pushing it.

I don't have the logging parameters memorized right now, but I'd like to log the air mass, EGR valve positions (high and low pressure), target/actual lambda values, and boost pressure (target/actual). In both the problem case and the normal case.

The problem with these engine control units is that they no longer have a turbocharger pressure control system, but instead regulate similarly to gasoline engines, using a lambda sensor. Therefore, it is possible that even with seemingly good performance improvements, problems may arise in the partial load range.

Thank you for the answers.

I was just reading the following:
Oil residue: 12.5 grams.
Maximum load: 80.0 grams.

Unfortunately, I haven't found any extended measurement blocks for the DPF differential pressure.

I'll find a quiet moment to perform the log recording with the AGR valve, lambda sensor, and boost pressure. It's difficult to record the error case because it's not reproducible.

I understand your information, Blackfrosch. I believe you. However, there's also an acquaintance with a Euro 6 MKB CUNA from the Skoda Club who was at the exact same place for software optimization as I was (on my second visit). He complains that he has absolutely no problems...
These are all details that make me question the capabilities of my hardware.

Do you know which of the AGR (Abgasrückführung) valves, in the measurement block of the AGR control unit, is the high-pressure recirculation valve and which is the low-pressure recirculation valve? EGR 1 or EGR 2?

Purely from a logical perspective, I would consider EGR 1 to be high-pressure (classic EGR) and EGR 2 to be low-pressure, but unfortunately, I don't know for sure. I'll take a look at our Octavia sometime this week.

You yourself mentioned that, even for you (despite the fact that you've been dealing with this issue for a long time), reproducing the error state is difficult. At the same time, you are 100% certain that your car is the cause, and the problem doesn't occur in your acquaintance's car. Everyone drives (completely) differently from each other. If you know him well, why don't you swap cars for a while? I bet his car is doing the same thing.

Since I'm personally interested in the history of HD/ND-Lambda-AGR, I'll see if I can create a list that you, your friend, or I (with a 150hp EU6 Octavia) can use to log data. Maybe we'll find something, although the new cars are all different – but not necessarily reproducible. The AGR system has 196(!) correction maps (not all are populated...), and it would take something extraordinary for all the active correction maps to produce the same result.

Good evening, everyone.
Unfortunately, I haven't had a chance to go on any logging trips yet. I had planned to do so for tomorrow, Saturday. But this afternoon, I stopped at a parking lot, made a phone call, and then turned the ignition back on after about 20 minutes. Then, the message "Transmission defective. Visit a workshop" appeared. Ignition off again, then on; everything is back to normal.
I have just read the error memory. Here are the results (attached). What do you think about this? In almost every control unit (STG), I'm seeing errors like "Data bus missing message," etc.

I've already had to drive about 30km again. Everything went smoothly.

The errors are 99% likely caused by flashing the ECU. It's perfectly normal.
Actually, you usually fix those errors immediately afterwards.