Slow turbocharger boost, actual value too low

Hi,

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

This morning, I took my very old VAG-COM device for a test drive.
Some rather strange measurement values resulted.
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 have occurred:

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), while the specified values according to VAG-COM are significantly lower.
Der Luftmassenmesser meldet weit überhöhte Werte selbst im Leerlauf ? KOhmic defect.


What I have done so far:
- Vacuum pressure measured after 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 carbon buildup on the VTG (Variable Turbine Geometry) mechanism and re-lubricated the shaft, as well as performing a general overhaul. Therefore, the turbocharger *should* be 100% okay (hopefully...).
- 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, fully retracted (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 Gear Restraint system] has not been present 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 even at idle. Either there's a defect with the mass airflow sensor (wrong type?), or there's a problem with the connector, wiring, or 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 values in that range. 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 is from the year 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 even at idle. Either there's a defect with the mass airflow sensor (wrong type?), or there's a problem with the connector, wiring, or 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. Therefore, 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, there is a leak. 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,

Try the shareware, but I agree with Roger, there's probably still a problem with it.
Was ist wichtiger: Did it happen suddenly or gradually? Can you rule out seemingly "stupid" mistakes, like reversed valves, AGR VTG shut-off valve (which should be noticeable)? It seems like vacuum is completely present. The aspect of the duty cycle is really interesting...

matthiasTDI96 wrote:

@Jens, Try out some shareware, really.

I emailed Rainer, or are they available elsewhere?

Quote:

"I agree with Roger; there must still be a problem." Did it happen suddenly or gradually?

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 like 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 check 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 stuff .

The area around the airport is relatively clear, as I recently installed the turbocharger and have checked everything thoroughly.
The LLK (likely referring to a coolant reservoir) itself might be leaking, and it's located in a somewhat hidden spot. I would need to press on it to check if it's leaking. 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 concept) is high because a portion of the mass/air simply disappears through the hole (which doesn't have 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, that's not explained: How could the turbocharger 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 Transportation Group) elevator, when in the default position in building 11, travel a significant distance until it reaches the limit switches (attention, 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 in 3rd gear there.
The turquoise (blue) line represents the target boost pressure. When driving at full throttle, the engine control unit (ECU) demands 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 boost pressure drops below the target value and continues to decrease even with full throttle. Eventually, this 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 program too, 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 LL (likely referring to a specific measurement or value) is usually around 2800 (target) and 4500 (actual).

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

Regards,
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 is causing the engine control unit to reduce the boost pressure in order to prevent 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 behave non-linearly when there's a defect, meaning the value might jump or increase disproportionately with the temperature."

You should still take a look at your LLK (liquid coolant), just to make sure that the cooling effect is really guaranteed .

Otherwise, definitely perform a pressure test on the charge air system 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 sensor (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 steady-state conditions, the boost pressure initially rises to the target value, albeit slowly. 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, the cycle can be repeated endlessly.
That is, the turbocharger builds up to a relatively normal boost pressure, 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 injection."

The intake air temperature was always between 20 and 25°C, and the pedal position sensor definitely reaches 100% when you demand full throttle.
The intercooler duct 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 problem?

Regards,
Jens.

Hello.

As you may have already noticed, your MSG is increasing the VTG too slowly to reach the minimum PWM value. Therefore, it closes 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.
Let's move away from VAG-Com now and take a look 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 it continues there.

Best regards, Jochen.

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

Please provide the German text you would like me to translate.
What is the LLT (Lowest Limit of Tolerance) value? Is it 20-20 degrees?
"Can it 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 Steam Generator) remains in the "three-motion" state?
At roughly half the LD (load distance), shouldn't the MSG (message) be clearly visible within the smoke boundary?

3. What is the duty cycle 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 (Likelihood-to-Action) score, which probably shows the same thing.