Low Turbo Pressure T4 ACV: Causes & Fixes

Hello everyone.

Since this is my first post in this forum, I'd like to briefly introduce myself: My name is Torben, I'm 32 years old, and I'm from Dortmund. I own a T4 (ACV, manufactured in 2000). I usually participate in another online forum dedicated to T4s, but I'm currently experiencing a very specific problem with my van and hope that I can get some helpful advice here.

"Some time ago, the bus was visited by a weasel. During this incident, the pressure hose between the boost pressure control valve (N75) and the wastegate pressure sensor was severed. The fault wasn't detected until approximately 200 km later, at which point it was rectified. All pressure hoses around the boost pressure regulation system were replaced, and the rest of the air intake system and wiring harnesses were checked for damage."

Unfortunately, the bus is still not working properly. After the "repair," it now suffers from too low boost pressure. The MSG (Main System Module) reports error code 17964 (boost pressure regulation limit exceeded, intermittent). During operation, this manifests as the turbocharger only engaging at approximately 2500 to 3000 RPM. Below 2000 RPM, where the turbocharger doesn't normally engage anyway, the driving performance is normal. Sufficient VCDS log files are available. Apparently, as a new user, I am not yet allowed to attach files, so here is a diagram for clarification: [Edit Admin: inserted into the post because it takes too long to load].

Actions taken so far: Mass airflow sensor (MAF), wastegate valve (N75), and all filters have been replaced; the intake manifold has been checked with a hose clamp (to rule out air leaks, a common T4 issue); the EGR valve has been cleaned; fuel injection timing has been checked; the exhaust system/catalytic converter has been inspected (test drive with the downpipe removed, same behavior); possible turbocharger bearing play and oil contamination have been checked. Additionally, a "Speed-Buster" module was found, which the previous owner had installed and hidden. I immediately removed it, as it is not compatible with the installed diesel particulate filter (DPF).

The workshop (which is an independent garage, not a dealership) now tends to believe that the turbocharger may have been damaged during those 200 km with the faulty boost pressure control (wastegate permanently closed). Specifically, it seems that the wastegate is no longer closing properly. Would this be a plausible explanation? Wouldn't operating at excessive pressure/turbo speed more likely result in bearing damage? Or is the wastegate designed in such a way that it can "bend" due to the pressure? In any case, the external linkage and pressure sensor appear to be okay and correctly attached; it's all standard, and there are no broken screws or anything like that.

I'm now considering whether there might be another plausible explanation for the error. To that end, I've studied various articles and forum posts, and I've carefully reviewed the logs again. I've noticed that the soot limit is consistently very low (lower than the torque limit) almost all the time. Is this a consequence of the low boost pressure, or could it also be caused by insufficient diesel injection?

Or, to put it another way: The technical article about the Mass Air Flow sensor (MAF) states: "In some TDIs, the smoke limit is not calculated from the MAF value, but from the boost pressure." - Which of these two applies to the ACV engine?

I am grateful for any helpful suggestions.


Torben.

Hello,

Sure, you can upload the CSV file directly here, for example.
/viewforum.php?f=40

Since your VCDS log is quite amusing, I can see the following possibilities at first glance:
a) You're trying to sabotage us and you're losing momentum.
b) You have a tuning installed that was done by a complete idiot.
c) You suffer from Restless Legs Syndrome.

a), b) or c)?

Because the "driver preference" setting for accelerator pedal sensitivity kicks in exactly within that range.

Best regards, Rainer.

Oh shoot, I obviously missed the upload forum . I'll be sure to use it next time.

Regarding your question, I believe a) is the correct answer, but not necessarily intentionally. It's more likely due to practical reasons (try doing a proper log run on a highway in the Ruhr area during rush hour...). However, I definitely have some other, better logs available, or I might create a new one. Which would you prefer? Should I keep it at 3000 RPM, or push it up to 4000? (It seems opinions are divided on this?) And what about MWB 3/8/11?

Hi.

Unfortunately, the log is not usable in its current form.
Sure, the gas cap remover was definitely necessary so you'll still have something to log tomorrow and won't have to ask about "affordable front bumper for T4," but these pedal-to-the-floor logs have the immense advantage of eliminating some inconsistencies.

Only when you push it to its absolute limit does it become clear what the machine is capable of, because what a person does is obvious: they simply press down fully.

So, take some time late in the evening, warm up the living room, and then unleash it on the hopefully less crowded highway.

I usually go logging at night as well.

Okay, please provide the German text you would like me to translate into English. I will only provide the translation.

You are right. The image was initially just meant to illustrate my problem (as you can see, there's a noticeable "turbo lag" below 2500 RPM). It's clear that more informative logs are needed for actual troubleshooting.

I only have a cheap cable from eBay myself. I'm borrowing the good Ross-Tech interface from my brother-in-law tomorrow, and then I'll post the necessary data here.

Thank you very much in the meantime!

Please include the value 11.4 in the log, which represents what the N75 does.

You can see what the EDC *would like* to achieve based on their desired boost pressure, but then you see what actually happens when you look at the actual boost levels.

Okay, please provide the German text you would like me to translate into English. I will only provide the translation.

I always log 3/8/11. In the diagram above, I omitted the N75 duty cycle, which was constantly at 12.8% = wastegate closed. It's actually always like that, except for the moment when you release the accelerator pedal.

I once posted a longer log in the upload section where you can see it: https://community.dieselschrauber.org/en/viewtopic.php?t=22208 (file N75-1.csv).

This is a somewhat longer highway drive, and while specific acceleration events weren't logged, there are a few segments where the car was running at full throttle. For example, around 880 seconds.

I will provide a new log entry that focuses on the essentials tomorrow.

Hello,

Regarding the first log: If you look at the area before the throttle valve, you'll notice that the limit set by air mass increases very slowly and only reaches the torque limit at 2500 rpm, which is where the throttle valve starts to open.

Are you absolutely sure that this "new" LLM is the right one?

"Please install a new, genuine mass airflow sensor, but not one from eBay, as the search function indicates. The air mass of 500mg/hub is only reached at 1700 rpm, which is quite late."

Best regards, Rainer.

Yes, I also noticed the issue with soot reduction. That's why my question is: Does the ACV (AdBlue Consumption Value) regulate based on the mass airflow rate or on the boost pressure?

However, regarding the mass airflow sensor (MAF) value: I've also driven with a fixed resistor instead of the MAF sensor, and that didn't improve anything either.

The new mass airflow sensor (MAF) is a Pierburg 7.22684.08.0, purchased from a local parts supplier. I'll see if I can borrow a working Bosch MAF sensor to compare.

Quote:

However, if we're talking about exceeding the LMM value: I have also driven with a diode instead of an LMM before, and that didn't improve anything.

I don't believe that. Have you ever looked at the actual value of the mass airflow sensor (MAF) with a diode?

If not, please rectify it. The car must have been making a lot of noise when accelerating, but it definitely moved forward quite quickly.

Best regards, Rainer.

I meant the same driving characteristics, of course. The value would have been different. It's around 900 with the diode, if I remember correctly. But I'd be happy to measure it again tomorrow.

And how much soot comes out of it? I don't know. It has a DPF (Diesel Particulate Filter). That's not exactly ideal at the moment.

I have uploaded a new log: https://community.dieselschrauber.org/en/viewtopic.php?p=157633

Darin sind u.a. zwei Segmente mit jeweils einer Beschleunigung von 1500 auf 4000 U/min im 3. Okay.

Marker 1: Equipped with a diode instead of an air mass meter, and the EGR control disconnected (to ensure that no exhaust gases are mixed in).

Marker 2: AGR and mass airflow sensor reconnected, so the system is back to its normal state.

I need to revise my previous statement; the driving behavior with the diode is actually better. The values also show this: Up to 2500 RPM, the boost pressure curve with the mass airflow sensor (MAF) is much flatter than with the diode. So, the "new" MAF sensor probably isn't working correctly after all?

Above 2500 RPM, the curves are largely identical. The maximum boost pressure is 1723 mbar from approximately 3400 RPM. Passing through 3000 RPM, the pressure with the diode is about 1650 mbar (or 650 mbar relative, ambient pressure today is 999 mbar). According to my information, this is slightly below the standard range (700-950 mbar).

Okay, so next time I'm going to replace the lambda sensor again, this time with a genuine Bosch part.

The turbocharger has been replaced again. It's now a Bosch unit. The driving performance is definitely better, although I get the impression that the car isn't performing to its full potential or what it should be capable of. The boost pressure at 3000 RPM is just under 1700 mbar. That's not a lot.

I have uploaded a recent 3/8/11 log to the upload forum. If you would like, I can also provide an 8/10 log.

Hello,

The boost pressure is too low. Is the turbocharger faulty, or is the wastegate not closing properly? Is the DPF clogged/blocked due to experiments with vegetable oil or something similar?

Best regards, Rainer.

No, not PÖL, at least not since 2007 when I bought the bus – I don't know what the previous owner put in it.

The DPF (Diesel Particulate Filter) has been installed since June 2009. Last week, the workshop temporarily disconnected it for testing and drove a round without the exhaust system – still no pressure build-up.

Are there any other ways to address the issue of the wastegate not closing properly, or does the turbocharger need to be immediately removed and disassembled?

Please check the pressure hose for the wastegate and its corresponding solenoid valve.