Boost Control Valve Airflow Ratio (02-04-2003, 18:22)

Hello TDI enthusiasts!
I would like to check the plausibility of some of the measured values that I recently recorded using VAG-COM.
Specifically, it concerns the valve opening ratio of the pressure control valve in my wastegate turbocharger (engine code: AGR).
Based on the setpoint/actual pressure profile and the duty cycle, it can be observed that the bypass valve opens further with an increasing duty cycle, and conversely.
During the analysis of a rather long driving cycle, I noticed that in my TDI, the duty cycle only changes within a sub-interval that utilizes approximately 2/3 of the full range from 0 to 100%. The actual range goes from 34.7% to 91.6%. It's striking that the area near 0% (=complete closure of the wastegate), which is necessary for rapid pressure build-up, is never reached.
Is this conservative target value setting a turbocharger protection mechanism designed to prevent excessive pressure surges? Why doesn't the control unit specify lower duty cycles to allow the turbocharger to spool up more quickly?
In welchem Intervall bewegt sich bei Euch das Tastverhältnis? Is that normal?
My question doesn't seem entirely unfounded, because I've created an expanded Excel table based on the measured values, and a chart that displays the control deviation and the duty cycle in the same graph over time. This allows you to clearly see when the control system is "helpless," which is always when the duty cycle reaches its "lower limit" (34.7% in my case) and the magnitude of the control deviation doesn't decrease (or even increases).
This condition was frequently reached in my driving cycle, primarily at very low speeds and high load.
Certainly, I expected that due to the low exhaust mass flow rate and the inertia of the load, the duty cycle would remain at its maximum for 2-3 seconds before a reduction in the control error would occur. However, sometimes, even after 20 seconds, the desired pressure still had not been reached .

Thank you for your experiences, suggestions, and opinions.

Regards,
Alex.

Hello everyone!
Today, I also logged the measurement blocks 3 (air mass) and 8 (fuel injection quantity limits) as an addition.
Under full load, the amount required for smoke reduction is usually just slightly above the amount required for torque limitation (although sometimes it's only very slightly above). As far as it seems, everything appears to be in order.
However, I've noticed that the actual values of the lambda (air-fuel ratio) in the lower RPM range closely follow the target value, even with a tendency towards larger overshoots, meaning an excess of air.
Compared to the reference values, the readings are also acceptable, but they tend to be closer to the lower limit of the range.
However, things look quite different above 3000 RPM; here, the air mass flow (or is it just the reading of the mass airflow sensor?) seems to collapse abruptly ! The lower target value at full load and n=3000 RPM is just barely reached, but with further increases in engine speed, the value drops. The boost pressure also decreases slightly, but not as dramatically as the mass airflow value. While the boost pressure can remain at approximately 1.8 bar from 3000 to 4200 RPM, the mass airflow value drops from approximately 800 to 600 mg/H. Something is definitely wrong, isn't it???
By the way, I treated the LMM with Loctite Cleaner last summer as a precautionary measure.
The car doesn't really show any significant weaknesses in any driving condition, but I just feel like it's missing that certain "oomph" it used to have.
It would be great if someone among you could come up with a rhyme about this behavior.

Certainly, I'd be happy to help. Please provide the text you would like me to translate from German to English.

Regards,
Alex.

donalexo wrote:

However, things look quite different above 3000 rpm, where the airflow (or is it just the measured value of the airflow sensor?) seems to drop dramatically ! The boost pressure also decreases slightly, but not as drastically as the airflow sensor reading

.

Hi Alex,

The most likely explanation, in my opinion, is that your LMM (likely referring to a mass flow meter) is no longer providing accurate readings at high flow rates.
As a result of the consequently reduced injection quantity, it may also not be possible to maintain the boost pressure at the desired level.

--> Clean the LMM (with brake cleaner spray) or replace it with a known good, functioning one .

Could the reduced air mass possibly be due to a faulty EGR valve that is not closing properly?

If the valve is supposed to be closed at full load, but remains open, the pressure of the exhaust gas stream can counteract the pressure of the compressor, preventing the desired amount of air from flowing through the mass airflow sensor (MAF). This results in a portion of the fresh air being replaced by exhaust gases.

Diesel-Dirk wrote:

Could the reduced air mass possibly be due to an EGR valve that is not closing properly? If, at full load, the valve is supposed to be closed but is still open, and the pressure of the exhaust gas acts against the pressure of the compressor, then the desired amount of air cannot pass through the mass airflow sensor. Consequently, a portion of the fresh air is replaced by exhaust gases.

Hi Dirk,

It's actually the opposite: the excessive boost pressure forces fresh air directly into the exhaust.
"Because this portion of air also passes through the mass airflow sensor (MAF) and affects the fuel injection amount!), but is not available in the cylinder for combustion, the engine will produce soot if the EGR system is leaking – a well-known symptom."

But surely the exhaust pressure is greater than 1 bar at full load. The cylinder pressure is over 50 bar. To compress the intake air, the pressure difference at the turbine's drive must be greater than the pressure difference at the compressor. Therefore, the exhaust pressure must be greater than the turbocharger pressure.

?????????????????????????

Diesel-Dirk wrote:

But at full load, the exhaust pressure is definitely greater than 1 bar.

Hi Dirk,

It is not necessarily the case that: if the exhaust pipe diameter is large enough, the average exhaust pressure can even be equal to the external pressure.

Quote:

The cylinder internal pressure is over 50 bar. In order to compress the fresh air, the pressure difference at the drive of the turbo must be greater than at the compressor. Therefore, the exhaust pressure must be greater than the turbocharger pressure.

However, the volume flow also plays a role in the energy balance of the charger: the hot exhaust gases flow much faster than the intake air.
As far as I know, a lower pressure difference on the drive side might then be sufficient.

But let's assume, for the sake of argument, that you are right: how do you explain the excessive soot at full load caused by a severely leaky EGR ?

Thank you for your answers.

In summary, two likely sources of error emerge, namely:
1. the mass airflow sensor, which provides readings that are too low at engine speeds above 3000 RPM.
2. a faulty EGR valve.

Consequently, I will disassemble the LMM and clean it with brake cleaner. Then, I will perform another measurement to verify any potential improvement in its performance (or lack thereof).
I might be able to borrow an LMM (lambda sensor) from a colleague's Beetle to perform a test with a different component. However, I don't know its specific part number yet, and I'm not sure if I can use that LMM in my AGR engine for testing purposes. As far as I know, it's a Beetle from the year 2000. Are there significant differences between the different LMMs?

I'll also be tackling the EGR valve. Hopefully, the removal won't be too difficult (experiences?). I'll try to clean it as thoroughly as possible from deposits (brake cleaner?), and if necessary, restore its free movement.
While I'm at it, I might as well take a look at the intake manifold. Hopefully, I won't get a nasty surprise after 130,000 km of engine exhaust fumes!

I will share my findings with you on this matter. However, it may take some time before I have the opportunity to do so.

However, I would like to offer another point to consider. Could it also be that the wastegate valve of the turbocharger is sticking?
The regulation often operates at the lower limit of its range, without a significant increase in boost pressure. Perhaps the valve no longer seals completely, or it is extremely stiff in its final range of travel, causing a sluggish and insufficient boost pressure build-up at very low engine speeds.
What do you think of this thesis?
Has anyone here ever "worked on" the wastegate valve of a turbocharger?
Can this be done without disassembling the charger? Are there any specific things to be aware of?

Thank you for your help.

Regards,
Alex.

Does anyone happen to have the part number for the mass airflow sensor (MAF sensor) that's compatible with my AGR engine?
I've seen various offers on eBay, but they all have different part numbers (where part numbers are listed).

Thank you.

Regards,
Alex.

@donalexo

If you're going to include links, please also provide the direct address.

Your eBay links are being routed through an advertising banner account!

This is not nice here!

SORRY SORRY


The link address is automatically inserted like that. I didn't know that.

Interestingly, the membership number (87948) is different from the one displayed on the visible banners on all other pages (86323).

Once we've sorted out the issue with the automatic links (I was quite surprised myself, as I wasn't aware of any "fault" on my part), I would especially appreciate topic-related posts!

Does anyone here know the part number for the mass airflow sensor (MAF sensor) for the AGR engine? Are there perhaps several different versions that are compatible with the engine?
In the online auction mentioned above, there are several vendors offering LMMs (likely referring to Lambda Oxygen Sensors) with completely different part numbers, and the product descriptions also include the engine code "AGR." Is this an error on the part of the vendor, or is it actually possible?

Thank you for your posts.

Regards,
Alex.