A4 AFN: Minimizing Performance Gap

Hi,

I spent the entire weekend browsing through this (and other) forums, but I've decided to post now.

I've been driving an A4 Avant TDI with the AFN engine for a few months. It has 122,000 kilometers (approximately 76,000 miles) and was manufactured in September 1996.

I am very satisfied with the engine, except for one small thing (it's my first diesel, and I'm not used to such low fuel consumption at all... ):

Under full load in 2nd gear, at approximately 2400 RPM, there's a slight dip in power – as I mentioned, it's just a small dip, but it's noticeable. After about 1600 RPM, when the boost kicks in, you can feel it. Then the power returns fully. It feels like a brief drop in boost pressure.
It's not noticeable when the engine revs up in neutral or in first gear, as it happens too quickly. In third gear or higher, you almost don't notice it (maybe a slight drop in third, but not in fourth or fifth), because it happens too slowly (and in my opinion, the boost pressure can recover quickly enough after the brief drop).
I can reach a top speed of just over 200 km/h, and the fuel consumption of just under 6 liters seems normal (with the air conditioning on). It's difficult to detect any rust.

Now, after reviewing all the articles and posts I found here, I've checked the VTG (presumably a valve or component). In my opinion, it seems okay. I can almost draw air through the tube with my mouth, up to the other end, or I can move the rod with my fingers (although the force of the return spring is quite strong - is that normal?).

Error memory is empty.

Do you need any more information? If it's the LMM (likely referring to a mass airflow sensor), I definitely won't replace it because it would cost too much for such a small problem. But if there's something I can do myself to fix it, I'd definitely like to try!
Sure, no problem. Here's the translation:

'Thank you in advance.'

Best regards,
mortl

mortl wrote:

Under full load in 2nd gear, at approximately 2400 RPM, a slight dip in power can be felt - as I said, it's just a small dip, but it's noticeable. After approximately 1600 RPM, when the thrust begins, this small dip is present. Then, the power returns fully. It feels like a brief drop in boost pressure. When accelerating quickly in neutral or in first gear, it is not noticeable because it happens too fast. In third gear or higher, you also hardly notice anything (maybe a little bit in third, but not in fourth or fifth). The error memory is empty.

Hi.

If the VTG threaded rod is slightly too short, you may experience strong pressure surges, especially in the lower gears, followed by a brief drop in pressure – you're likely feeling exactly that.
If you have an LDA (Liquid Crystal Display) installed (or can temporarily connect one), you can also see the process.

Solution: Adjust the VTG rod in increments of no more than 1/2 revolution longer.
A slight amount of lag at low RPMs might be present, but it should make the throttle response feel "softer."

Hi,

That sounds absolutely logical... I also get the impression that the initial power output is a bit 'stronger' than that of other AFN engines (I was able to test two of them for comparison at a friend's place).
Then I'll try to soften it a bit... hmm, although well... maybe just a little ..
Thank you for the quick (and helpful) response!

I had something else in mind, but this is just a thought experiment.
Full load - VTG (Variable Turbine Geometry) at maximum, boost pressure is building up -> when this reaches its maximum (VTG has already been adjusted), there's a leak somewhere in the intake system, a small hole that only releases air at maximum boost pressure => loss of power => the VTG goes back to maximum and can just compensate for this small air leak, resulting in 'normal' power again.

However, there's no hissing sound or anything like that, and the car doesn't make an excessive amount of noise. (Is that right?)

Your explanation sounds much more plausible!

Best regards,
mortl