G3 AFN Power Loss: Troubleshooting Guide

Hello,
My newly acquired G3 variant with an AFN engine, manufactured in August 1997, is still having problems. Since I bought it, it has never run as I expected. There's a loss of power at all engine speeds. It always feels like the engine is sluggish and somehow being held back. Acceleration is poor in higher gears (4th and 5th). There's a significant drop in power when encountering even a slight incline. A new fuel temperature sensor was installed shortly after purchase due to a faulty one. I don't think the problem lies with the fuel pump.
Using VAG-COM, I've already investigated quite a few things. There are no error codes stored. I checked groups 011, 008, 004, 003, and basically everything that Ulf mentioned in his diagnostic suggestions regarding VAG-COM, but I haven't found anything specific. Except that the mass airflow sensor value decreases with engine speed. At 2000 RPM, I have a value of 920-950 mg/h, and at 4500 RPM, it's around 820 mg/h. To check the performance with maximum airflow, I installed a diode, but it didn't make a noticeable difference. I replaced the fuel filter...no change. Because of fluctuating boost pressure, the control valve was replaced, and the turbocharger was replaced with a GT1749VA. These measures also didn't lead to success.
Oh, so... the AGR (Automatic Geographic Recognition) was deactivated immediately after the purchase!

I'm a little unsure about what I should check next. Should I check the fuel injector? Could the intake manifold be clogged?

I would be grateful for any help.

Greetings.
Reuselkopp.

You wrote that you were experimenting with a diode as a mass flow meter.

Have you observed the Russfeld boundaries both before and after?

In addition, a diode can also cause drops in the mass airflow sensor (MAF) value at a certain engine speed – I have personally measured this with my own diode.

In order for us to provide a definitive answer, we need to gather some logs – ideally, logs from devices 003-008-011 all at once, and in various states (e.g., multiple DZR trips).

Hi dieslmartin,
Okay, I had already looked at the Russfeld limit. At 3000 RPM, the driver's desired value was around 43 mg/H, and the Russfeld limit was around 42 mg/H. The torque limit is always around 42 mg/H. I had also considered reducing the output by the Russfeld limit, but the values don't seem to indicate that. I don't have any readings from the diode yet, but I'll check it again tonight.


Greetings.
Reuselkopp.

Did you really check all the boost hoses for leaks?!
I also experienced a similar strange loss of power with my Ibiza (AFN) once, and the maximum speed I could reach was only 180 km/h!

Ultimately, I replaced a whole bunch of those boost pressure hoses just to be safe! One of them had tiny holes in it (likely caused by a weasel), and the connector at the engine control unit was also frayed. I replaced them all, and now the car is running perfectly again.

Best, Boro.

reuselkopp wrote:

I'm a little unsure about what I should check next. Could it be a fuel injector problem? Or is the intake manifold clogged?

Yes, and everything else the article has to offer.

Unfortunately, I can't give you a specific tip like "it's probably xy, check there first."

Hello,
Unfortunately, I haven't had a chance to adjust the logs yet. I've now adjusted the turbo to be a bit more aggressive, meaning I shortened the adjustment lever. Now, at full throttle, the boost control valve's frequency is about 90%. I got this setting from a thread by Rainer, where he posted it as a tip. What I don't quite understand is that I'm now consistently above the target boost level in the lower RPM range, up to 2000 RPM. Above 1900 RPM, I have full boost, which regulates to about 1.0 bar with almost no overshoot. However, this adjustment hasn't changed the weak performance of the engine. Does the excessively high boost pressure in the lower RPM range have any negative effects? There are no error codes stored, and when I look at group 008, the torque limit is always the lowest, at about 42 mg/H. The soot limit is higher, as is the driver's request.
"This weekend, I'll check the fuel lines for any blockages and take a look inside the intake manifold to see if there are any large deposits."

Greetings.
Reuselkopp.

reuselkopp wrote:

I've now adjusted the turbo to be a bit more aggressive, meaning I shortened the adjustment lever.

What I don't quite understand is that I'm now consistently above the target value in the lower RPM range, up to 2000 RPM. }
Well, as far as I know, the boost pressure control system initially starts with a pre-programmed starting value from a lookup table based on engine speed, target pressure, and duty cycle.

If the desired pressure is not achieved, adjustments are made – although not always to the extent that the deviation is completely eliminated.
I'm going to refer to this here as a "soft" regulation, although I'm not familiar with the technical terms -> @ Experts: please correct me!

This means that a difference remains between the desired and actual values, and the magnitude of this difference depends on how inaccurate the length of the VTG rods is compared to the nominal value.

In addition, the control strategy of your MSG appears to be "softer" below 2000 rpm than above that speed.

I'm currently fine-tuning my VTG (Variable Turbine Geometry) vane positions for chip tuning. Apparently, the load-dependent (LD) control strategy in the ASZ engine is much "softer" than I remember from the AFN engine, meaning that adjustments to vane length are less effective in the ASZ. I think this should be added to the VTG technical article.

Hello,
Does the difference between the target and actual values in the lower RPM range become noticeable in terms of power output?

Regards,
Reuselkopp.

Hello,
If the boost pressure is too low, less fuel will be injected.
Best regards,
Günther.

Günther wrote:

Hello, If the boost pressure is too low, less fuel will be injected. Best regards, Günther

Hi, but he said the soot limiter isn't working.

Rainer K. wrote:

Günther wrote: Hello, If the boost pressure is too low, less fuel will be injected. Best regards, Günther Hi, but he said that the soot limit doesn't apply...

. . . and "he" is probably a clueless person, although he himself asked the initial question . . . but to whom

Quote:

Does the difference between the target and actual values in the lower RPM range become noticeable in terms of power?

If I am being referred to:
Yes, as usual -> as long as the soot limit is at its lowest.

Hi,
"I'm saying this so that no one here gets overly excited or argumentative. My boost pressure is always above the target value in the lower RPM range, up to 1900 RPM! I'm already seeing an overboost of about 300 mbar at 1200 RPM."

Question for everyone... am I correct in assuming that this difference between the target and actual values in the lower RPM range has no influence on my reduced power output?

Reuselkopp.

Hi Ulf,

I actually meant Günther.

@reuselkopp: The slightly higher boost pressure at low RPMs shouldn't matter.

Best regards, Rainer.

Hello,
If the boost pressure is too low, less fuel will be injected.
Best regards,
Günther
Hi, but he said that the soot limit doesn't apply...

Hello Rainer,
Okay, but who will 'bite' then?
Best regards,
Günther.

Hello,
I would say it doesn't exceed any limits! At least, the values in group 008, boost pressure, are also perfectly fine, and so is the mass airflow sensor (MAF). Now, after increasing the turbo's boost, the MAF value starts to drop slightly around 4300 RPM.
My little engine is just stubborn. But strangely, it's not always like that; occasionally, and very rarely, the car runs smoothly (until the next gear change). Unfortunately, it's impossible to predict when or why it might run well.
Ich meine der Motor hat entweder zuwenig Luft oder zuwenig Sprit. Aber wenn er zuwenig Luft hätte (eventuell durch eine zugeschlammte Ansaugbrücke) dann würde er doch auch rußen. Oder?
Later, I'm going to disconnect the main tank and run with an auxiliary tank in the engine compartment, and I'll remove the filter to see if the problem might be with the fuel tank system and hoses. It's probably just the hoses, as I already drove without the fuel cap yesterday, but without success.

Reuselkopp.