High Boost Pressure (AFN) Explained

Hello,

"I installed a GT 1749VA in my car quite some time ago... It worked pretty well overall. I occasionally had a problem where it would build up to 1.5 bar in 5th gear, but otherwise it ran normally (except that it would go slightly above 1 bar at high RPMs, which is normal)."
"I've now installed the 0.216 injectors and had the engine remapped. Since then, it only boosts (to 1.5 bar) in first and second gear. (In second gear, there are overboosts up to 1.7 bar)."
Starting from the 3rd gear, I have to regulate the turbo boost pressure with my foot because it exceeds 2 bar!

The VTG rod is quite easy to move. I can move it with my hand without any problems, and there's no clicking or other noises.

I assume the solenoid valve is working correctly, since it seems to be regulating the pressure in the first two gears.

What else could it be?


I read all the values using an LDA.

Perhaps your control system is too slow, or the vacuum pressure is not sufficient to properly adjust the VTG. Of course, the LDA could also be malfunctioning.

or simply incorrectly calibrated .

Who voted on your MSG, and were they aware of the nozzles?

Best regards,

Hello.
Look, he had the same problem.
/viewtopic.php?t=5194&highlight=problems+vnt17
Best regards,
Dirk

The chip is tuned to the injectors. He also knew about the turbo. It's not that either. What's making me suspicious is that it's regulating in 1st and 2nd gear...

Are the exhaust emissions significantly higher from the third gear onwards compared to the first two? I'm planning to try installing a different turbo soon.
Then, at the very least, we should be able to rule out the turbo. I don't want to start messing with anything on the linkage unless I know for sure that's the problem...


The LDA is working fine; without the chip, it showed me a reading of 1 bar.

It can't be due to the vacuum, right? As soon as I start the engine, the linkage should be pulled all the way in... If it were due to that, it would already be failing, wouldn't it?

So, the vacuum creates the pressure buildup. Too little vacuum would cause a low LD (lightness defect), or that's how I can explain it.

Have you created a log file for security purposes? Can you rule out the N75 as the cause? You could try swapping in your EGR valve as a test.

Let's try to explain why this only happens when shifting to higher gears. The charger needs a certain amount of time to build up the required load. What if you shift to a higher gear before this load is reached? Then the system doesn't adjust differently, but only starts regulating in higher gears when the regulation is too slow. I don't see any other explanation; otherwise, it would need a gear recognition system.

I suspect that the tuning software might simply be incorrect. Surely, your problems could be resolved by adjusting the pre-injection map. Or perhaps the injectors haven't been properly accounted for...

Why don't you take your problem to a tuner? Your turbo should be fine. At most, you might want to experiment with the length of the VTG linkage. But you probably don't want to do that.

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So, the vacuum creates the pressure buildup. Too little vacuum would cause a low LD, or that's how I can explain it.

"When you start it, the rod retracts... And since it operates with vacuum, I assume the maximum vacuum pressure occurs during starting."

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Just to be safe, did you create a log file?

My laptop is acting up right now. When it's working again, I'll do one.

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Can you rule out the N75 as the cause? You could, as a test, use your EGR valve.

He's already adjusting it in the second gear. It overshoots to 1.65 bar, then drops back to 1.5 bar.

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Let's try to explain why this only happens when shifting to higher gears. The charger needs a certain amount of time to build up the required load. What if you reach the point before the overswing occurs, and you already shift gears?

Please provide the text you would like me to translate from German to English. I need the text to be able to translate it.

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Why don't you take your problem to the tuner?

Since he lives quite far away and the problem already existed in a basic form there, I assume he can't do much about it with software.

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Your turbo will be fine eventually.

I've experienced this issue before, but it wasn't severe then, as the pressure only reached a maximum of 1.5 bar. I suspect that it's now occurring more frequently due to the increased stress.

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You might experiment with the length of the VTG rod, but you probably don't want to.

I'm considering working with the shaft, but I want to do that only after I'm sure that's the problem. Besides, I have to remove the turbo every time to do that... (engine mounted longitudinally).

cksIT wrote:

1) Quote: So, the vacuum creates the pressure buildup. Too little vacuum would cause a low LD, or that's how I can explain it. "When you start it, the rod retracts... And since it operates with vacuum, I assume the maximum vacuum pressure occurs during startup." 2) Quote: Can you rule out the N75 as the cause? You could, as a test, use your EGR valve. He's already adjusting it in the second gear. It overshoots to 1.65 bar, then it drops back to 1.5 bar. Quote: Let's try to explain why this only happens when shifting to higher gears. The charger needs a certain amount of time to build up the required load. What if you reach the point before the overswing occurs and already change gears?

1) The vacuum is always approximately the same, but the N75 valve allows more air through as long as the LD (load demand) is below a certain value, which causes the rod to be fully extended during pressure buildup. If there is no vacuum, the rod is extended less, the pressure buildup occurs later, or remains too low (which, I believe was due to the pre-injection characteristic map). It's roughly like I described.

2) Then it's probably because it oscillates at the same speed, but the oscillation is also higher due to the increased load. In that case, the problem is either related to the load condition (in connection with the VTG linkage) or the software in that speed range.

I still don't believe the loader is broken, but rather it's either a software issue or a problem with the adjustment of the VTG linkage.

It would be best to have a log with this information. This log should include data on the duty cycle and boost pressure. If the turbocharger is "broken," the duty cycle should at least show some signs of overboost.

Does the turbo really have to be removed? That makes the precise adjustment quite complicated. You just have to "simply" turn the knob...

cksIT wrote:

Starting from the 3rd gear, I have to regulate the boost pressure with my foot because it shoots over 2 bar! I read all the values using an LDA.

Hi,

To check if the turbocharger is not being oversaturated by the exhaust gas flow (which can sometimes only occur in higher gears due to software limitations), try disconnecting the vacuum hose from the turbocharger control valve and temporarily sealing the end.
Then, gradually accelerate (so that the MSG doesn't go into emergency mode due to a low-speed regulation error) to approximately 50 km/h in third gear, and then, as the engine speed continues to increase, gradually apply full throttle. Around 3500 rpm, you should experience the usual low-speed torque.

If it also reaches 2 bar (which, in my opinion, is a nightmare for the poor turbocharger and the cylinder head gasket), then the turbocharger is too small (or the VTG is so burnt that it doesn't open fully) or the injectors are too large -> you have to figure it out.

If the leakage rate remains within an acceptable range, there might be something we can do to improve the leakage rate control (software, pneumatic system).

2 bar are reached (IMO a nightmare for the poor VA loader.

From what boost pressure does a turbocharger actually start to spool up during a single event?
Do overshoots cause damage?

Best regards.
Peter.

neubaupe wrote:

From what boost pressure does a turbocharger actually start to spool up during a single instance? Overswings can cause damage?

It depends on the flow rate it's delivering: the more the motor consumes due to its speed, the higher the turbocharger will spin at a given pressure.

For a VA (presumably a vehicle or engine) that only produced a single over-rev of 2 bar at 4000 rpm, I certainly wouldn't spend any more money on it... but that's just my gut feeling; maybe these things are actually more robust.

Regarding durability: I can't imagine that a pressure of 2 bar for half a second would immediately ruin it if it's normally running at 1.5 bar. However, I lack the technical expertise, but I believe that both the pressure and the duration need to be considered. I think that running at 1.5 bar for a longer time is probably not worse than briefly reaching 2 bar.

Okay, I just did a test run. I simply disconnected [something]. At first, it seemed to work well, but the moment the boost pressure should have kicked in, it barely reacted (which makes sense, since it was completely open). When I gave it full throttle, I got 1.3-1.35 bar of boost. I suspect that might be normal?

And since he's able to properly adjust the timing in the second gear, I'm increasingly convinced that the problem is simply a misaligned shaft.
Furthermore, it also regulates the pressure at higher speeds. When I drive with a light throttle, it increases to 1.5 bar and then drops by about 0.2 bar (and correspondingly, if I ease off the throttle, it goes from 1.3 bar to 1 bar, etc.).
It just reinforces the suspicion that he simply can't manage things anymore.
When I connected the hose while the engine was running, the linkage quickly reached its full travel. So, it's definitely a common issue.

Does the boost pressure sensor report the correct pressure to the engine control unit (ECU)?

I've now extended the rod by one rotation, and now it's also adjusting in 3rd gear. It's still not adjusting in 4th gear, though.

"Since it sometimes regulates properly without the chip and sometimes goes up to 1.5 bar, I'm a bit at a loss now... what could it be? I don't want to be covering up any problems."


It still hardly produces any soot.

cksIT wrote:

I've now extended the rod by one rotation, and now it's also adjusting in 3rd gear. In 4th gear, it still isn't adjusting... "Since it sometimes regulates properly without the chip and sometimes goes up to 1.5 bar, I'm a bit at a loss now... what could it be? I don't want to be covering up any problems." It still hardly produces any smoke...

It's best to check the data logs. Specifically, are you using the original MAP sensor? What sensors does VAG use in that case, and do they also go up to 2.0 bar? My problem was that the pressure was sometimes higher than what my MAP sensor could measure, which resulted in massive boost pressures in the higher gears and unpleasant overshoots in the lower gears (once the measurement limit is reached, the engine control unit registers these overshoots and no longer allows them). The check engine light doesn't necessarily have to be on if the target boost pressure is 100 mbar above the actual value (the sensor's measurement limit). However, with a data log, you would know for sure whether it's actually reaching its target value or if the measured pressure is too high and the target value in the engine control unit is not being reached.

Question / Just an idea.

It is not known whether the STG (regardless of the model or year of manufacture) has...
how the control is influenced by the gang.

Best regards.
Peter.