Boost Pressure Slow, Target Value Too Low

Hi,

It's starting to feel a bit mysterious to me with my AFN.

This morning, I took my very old VAG-COM device for a test drive.
That resulted in some rather strange measurement readings.
I still have an analog boost pressure gauge in the car, and the values it displays there (as far as one can visually compare) correspond to the readings from VAG-COM.


The following errors are occurring:

Block 11:

Target boost pressure is generally too low; the boost pressure builds up slowly and sluggishly.
Above 1700 mbar (absolute pressure), it hardly increases. Under full load, the control unit enters a fault mode (error code 575) after a few seconds. This makes logical sense to me.

The duty cycle of the N75 rarely exceeds 50%, and even under full load, it doesn't go above 70%, despite the lack of boost pressure.

Block 3:
The displayed air mass is generally too high .

VAG-Com displays values between 8500 mg/h (idle) and 12500 mg/h (at low load), which are significantly higher than the specified values according to VAG-COM.
Der Luftmassenmesser meldet weit überhöhte Werte selbst im Leerlauf ? KOhmic defect.


What I have done so far:
- Vacuum pressure measured downstream of the check valve: -950 mbar (This is a high value, isn't it?)
- The turbocharger was sent to the repair shop because I detected increased bearing play. The technician reported a sooty VTG (Variable Geometry Turbine) adjustment and replaced the bearings, as well as performing a general overhaul. The turbocharger should therefore be 100% functional (or at least, it should be...).
- N75 valve replaced (well, it worked reasonably well for a very short period after the replacement, but now it's back to how it was before).
- All vacuum lines were inspected, and some were replaced as a precaution due to minor visual defects.
- VTG adjustment moved manually - very smooth, no sticking.
- Suction applied to the vacuum sensor - Rod moves smoothly.
- VTG rod, maximally shortened (unfortunately, it had no effect).


What further investigations are needed to determine the cause of the lack of boost pressure?
(Maybe swap the LMM with a diode?)
(By the way, the AGR [automatic gearbox restriction] has not been active for 140,000 km.)

Regards,
Jens.

If VAG-Com is actually displaying the numbers mentioned, something is wrong. The mass airflow sensor (MAF) values are already off by a factor of ten, as you probably noticed. Either there are typos, or the VAG-Com installation is faulty.

Apart from that, the magnitude of the value doesn't seem right either. No TDI engine I know of reaches 850 mg/H in idle. Either there's a defect with the MAF sensor (wrong type?), or there's a problem with the connector, wiring, or the control unit.

Here's a construction site for you. If I may make a guess , I imagine that the engine control unit (ECU) is trying to regulate the air mass via the turbocharger pressure, and is in a critical state. Perhaps that's why the pressure is low?

Roger wrote:

If VAG-Com is actually displaying the numbers mentioned, something is wrong. The mass airflow sensor (MAF) values are already off by a factor of ten, as you probably noticed. Either there are typos, or the VAG-Com installation is faulty?

"The VAG-COM has always displayed this kind of value. It's definitely not due to a faulty installation, since the other values seem correct. It's probably due to the version, which is from the US and I believe dates back to 1999. Maybe it's an old bug or something."
The target values are also off by a factor of 10, which is at least consistent.

Quote:

Apart from that, the magnitude of the value doesn't seem right either. No TDI engine I know of reaches 850 mg/H in idle. Either there's a defect with the MAF sensor (wrong type?), or there's a problem with the connector, wiring, or the control unit.

Well, those 850 mg/h readings probably aren't a real value. I suspect there's a defect in the mass airflow sensor, but the question is how the engine control unit (ECU) reacts to those "readings," and whether there's a connection to the slow build-up of boost pressure.
Unfortunately, I don't have any comparison data from times when the AFN was still working perfectly .

Regards,
Jens.

Jens 16syncro wrote:

Well, those 850 mg/h probably aren't a real value. I suspect there's a defect in the mass airflow sensor, but the question is how the engine control unit (ECU) reacts to such "readings," and whether there's a connection to the slow build-up of boost pressure. Unfortunately, I don't have any comparison values from times when the AFN was still working perfectly.

After a moment of thought, I have to refute my own suspicion, because even with the fake LMM diode producing 1250 mg/H, normal boost pressure is still built up even when idling. So, I think you can rule out the LMM as the cause.

I'm not entirely sure I trust your VAG-Com; it seems unreliable for diagnostics. Have you tried a 409 shareware version? It should work with older interfaces. Otherwise, maybe Rainer has another suggestion. However, updating to a current version would definitely be a good idea... I'd get that sorted out first.

Has the charge pressure control valve been checked yet?

According to what VAG is showing, there is a leak. Please check the entire section from the turbocharger -> intercooler -> intake manifold.
I suspect you'll find what you're looking for there.


Regards,
voss

voss wrote:

According to what VAG is showing, something is leaking. Please check the entire route: Turbo -> IC -> Intake manifold. I suspect you'll find what you're looking for there. Regards, voss

Please explain that to me.

@Jens,

"Really try out the shareware, although I agree with Roger that there's probably still an issue there."
Was ist interessant ist, ob es passierte plötzlich oder allmählich? Können Sie "dumme" Fehler wie verwechselte Ventile, AGR-Ventil, VTG-Absperrklappe (was eigentlich auffallen müsste) ausschließen? Es scheint, dass der Unterdruck vollständig vorhanden ist. Das mit dem Tastverhältnis ist wirklich interessant...

matthiasTDI96 wrote:

@Jens, Try out some shareware, really.

I emailed Rainer, or is he also available elsewhere?

Quote:

I agree with Roger; there must still be a problem. Was ist passiert? War es sudden or gradual?

gradual, initially subtle loss of boost pressure, followed by more frequent limp mode occurrences.
I was therefore also suspecting the turbo, but it seems it's not the problem now. I've had a similar turbo issue before, which is why I quickly suspected it. I also thought I detected some excessive play in something.

Quote:

"Silly" mistakes, such as reversed valves, AGR VTG bypass valve.

There is no AGR (Abgasrückführung) system present (neither an electric valve nor any piping or other components), so there's nothing to swap out.

What is a diverter valve? As far as I know, the AFN model doesn't have one.

I'm trying to implement logging, which isn't so easy with this old software .

The area around the airport is quite busy, but it's manageable because I recently installed the turbo and have checked everything thoroughly.
The LLK (coolant reservoir) itself might be leaking, and it's located in a somewhat hidden spot. I would need to put pressure on it to check if that's the case. In any case, you can't hear anything (like a hissing sound) if you put your ear near the engine compartment.

Regards,
Jens.

Quote:

Please explain that to me.

What am I supposed to explain?

The value of LMM (likely referring to a specific measurement or parameter) is high because a portion of the mass/air simply disappears through the hole (which doesn't need to be large).
Dadurch hat man auch nicht den geforderte Laderdruck. Lt. Display as well.
The problem is not rust, given the high mass. Only the AGR valve is faulty.


Regards,
voss

"No, it's not explained: How could the turbocharger possibly suck in such a huge amount of air at idle? Over 1000 mg/h is a lot, isn't it? However, it's not impossible that there's a leak somewhere."

@Jens:

Regarding the LD control valve:
Does the VTG (Vertical Transport Gear) in the default position (MWB 11) move up significantly until it hits the upper limit switches? (Caution: Watch out for your fingers!)

Okay, here are two log files:

Measurement block 11:


It concerns the area between the two vertical green lines.

I conducted a full-load test on the third gear.
The turquoise (blue) line represents the target boost pressure. When driving at full throttle, the engine control unit (ECU) demands the maximum boost pressure.
The yellow line represents the measured boost pressure.
As you can see, the yellow line lags behind the requirement and slightly overshoots the target value. But then things get mysterious...
The turbocharger pressure drops below the target value and continues to decrease even when the accelerator is fully pressed. This eventually leads to the engine entering a limp mode.
Since the green curve (the duty cycle from the N75) corresponds to this behavior, the control unit "wants" it to be this way. Therefore, the error must be located somewhere in the sensors/actuators.

Does anyone have any ideas about this?

And here's another log from measurement block 3:


Here, you can read the remarkable readings from the air mass sensor (yellow line).

Hmm... let me think...

Regards,
Jens.

Hi Jens,

Don't worry about the LMM, I have the same software and the values are normal.
The fact that the actual value of the mass airflow sensor is higher than the target value is due to the missing EGR (Exhaust Gas Recirculation) system.
For me, the target value is usually around 2800, while the actual value is around 4500.

Unfortunately, I don't have any ideas about the boost pressure behavior at the moment either.

Greetings,
dieter

Hello Jens,

Your boost pressure control behavior is truly very mysterious.

One possible explanation, in my opinion, is a faulty charge air temperature sensor that is providing an implausibly high value, which in turn causes the engine control unit to reduce the boost pressure in order to protect the engine from overheating.

"Could you log the intake air temperature during a full-throttle acceleration in a high gear? Sometimes, these temperature sensors can exhibit highly non-linear behavior when there's a defect, meaning the value might jump or increase disproportionately with the temperature."

You should really take a look at your LLK sometime, just to make sure that the cooling effect is actually guaranteed .

Otherwise, definitely perform a pressure test on the charge air duct to ensure its tightness.

Good luck and success...

Regards,
Alex.

Hi,

I now have a "new" software, but unfortunately, the measurement result remains the same.

In addition, the Lambda Measuring Module (LMM) has now been replaced with a diode, and the VAG diagnostic tool consistently displays a value of 1275 mg/h, as predicted by theory.

Unfortunately, this doesn't affect the build-up of boost pressure at all.
The described pressure pattern is still inexplicable to me.
Under constant load, the boost pressure initially rises towards the target value, although at a slower rate. However, after a few seconds, the engine control unit (ECU) reduces the actual boost pressure again (to approximately 0.5 bar above atmospheric pressure), even though the target value remains at 0.95 bar.
Logically, the engine control unit (ECU) will eventually enter a limp-home mode with error code 575.
If you briefly reduce the power output, you can repeat the cycle indefinitely.
That means the turbocharger builds up to a somewhat normal pressure level, and then it's reduced.
"When the turbocharger pressure drops, there's a lot of soot coming out of the exhaust. That makes sense, because there's not enough air with too much fuel being injected."

The charge air temperature was always between 20 and 25°C, and the pedal position sensor actually reaches 100% when you demand full throttle.
The intercooler circuit is sealed.
The measurement data block showing the injection quantities (actual, torque limitation, and soot limitation) also appears "normal," meaning that the actual injection quantity reaches the torque limitation.

What values should I log to get closer to identifying the root cause of the problem?

Regards,
Jens.

Hello.

As you may have already noticed, your MSG is increasing the VTG too slowly towards the minimum PWM value. This causes it to close so late that the LD continues to decrease, even though it should already be increasing.

It's also strange that this is something the MSG actually wants.
I'd like to move away from VAG-Com now and start looking at the actual sensor signals (Oscar).
Perhaps the LD pressure sensor is malfunctioning, providing values that don't reflect reality or are so noisy that the MSG gets confused and displays incorrect average values in the diagnosis that don't actually exist.

Perhaps there's more to it.

Best regards, Jochen.

Okay, there are at least three things I don't understand about your AFN.

Please provide the text you would like me to translate from German to English.
The LLT is between 20 and 20 degrees?
Can it not go any higher? That seems unlikely. I can easily achieve over 50 degrees Celsius even with the current weather conditions. Or were you referring to a DELTA of 20-25 degrees?

2. The LD (low-pressure alarm) drops to 0.5 bar, but the MSG (main safety valve) remains in the open position?
At approximately half the load capacity (!), shouldn't the MSG (Message Signal Generator) be clearly visible within the smoke suppression area?

3. What is the on/off ratio of the LD control valve?
Recently, there was a lengthy discussion about this, and I don't want to get involved in it myself, given my lack of experience. But are you sure that the logged key ratio actually represents an active reduction in the LD?

@Jochen:

He has a separate LDA (likely referring to a specific type of display or device), which probably shows the same information.