Pressure Test: Turbocharger Boost System

Hello,

I have the feeling that my turbo isn't working correctly, but it's producing a lot of smoke. I read an article by Ulf on systematic troubleshooting using VAG-COM and logged the data. One thing immediately stood out: even at idle speed, the target pressure (1.275 mbar) and the actual pressure (979 mbar) are significantly different. This was almost the case throughout the entire test drive, with a gap of up to 550 mbar between the target and actual values. When I then had the opportunity to hold the accelerator pedal down in 3rd gear from 1,400 - 3,500 rpm, the boost pressure built up to 1,999 mbar and settled at the target pressure, which is what it should do. However, as soon as I release the gas even briefly, for example when shifting gears, the boost pressure immediately collapses (-600 mbar in 0.5 seconds, which is needed for shifting). Shouldn't a boost pressure that has already been built up remain at a certain level for a while, even if there are no more exhaust gases driving the turbo?

IMO, this suggests a leak in the turbocharger pressure system – but I can't find any!

Is there another possible explanation?

Please excuse my amateur attempts at technical explanations; I come from a business background and am currently trying to learn about these things as a hobby, and this diesel is also my first one.

PS: As far as I know, the car is not chipped, but I have covered the EGR valve with a small piece of metal. Of course, I can provide the log files.

Hello,

Your description is actually typical of a leak in the turbocharger system or a stuck-open EGR valve.
How long has that piece of metal been there? Are you sure it hasn't rusted away?

What is your oil consumption like?

"Why don't you remove the intake pipe from the turbocharger and check the axial play (it shouldn't be noticeable) and the radial play (it should be minimal; the impellers must not rub against the housing under any circumstances)."
What is the mechanism behind your turbocharger control system?
Do you have a turbocharger with a wastegate or one with a variable geometry turbine (VTG)?
A wastegate valve might be stuck open. In a variable geometry turbocharger (VTG), the rubber hoses that control the vanes could be damaged or leaking, or the solenoid valve responsible for controlling boost pressure may have failed.
First, check what type of turbocharger you have.

Best regards, Rainer.

Were different turbochargers installed in the AEL engine?

Sure, here is the translation of the text from German to English:

'MFG' stands for 'Mit freundlichen Grüßen,' which translates to 'Sincerely' or 'Best regards' in English. It is a common way to end a formal letter or email.

Good evening everyone!

The oil consumption is approximately 1.5 liters between service intervals, which translates to roughly 15,000 kilometers per change.

I deactivated the AGR (Automatic Gear Restrictor) in May 2003 at a mileage of 209,000 km, which was about 6 months and 12,000 km ago. I used a thin copper sheet for this purpose (perhaps the material isn't heat-resistant enough?).

Unfortunately, I don't know what type of charger was installed in my car. However, I believe I once read in this forum that the AEL model didn't have a VTG turbocharger, but rather a wastegate turbocharger. That's why I had ruled out a difficult-to-operate VTG.

I'll continue searching and will keep you updated.

Thank you in advance for the tips!

Hi,
The AEL engine has a wastegate turbocharger. However, in my (AAT) engine as well, the pressure drops immediately when I release the accelerator pedal. This is more noticeable at low RPMs than at high RPMs. How clearly this manifests during the gear shift you described would need to be observed specifically.

Deleted upon request.

@diesel.gustav

"But the target boost pressure is being displayed by VAG-COM, and the program isn't just making it up; it's reading it from the values stored in the engine control unit. In other words, if that's the case, then the programming of my ECU must be incorrect, right?"

Best regards,

Hello,

Could it be that you saw the pressure at 1200 mbar just above the idle speed when accelerating (which would be okay), and is Gustav referring to the value at engine idle?
Does the turbocharger shaft now have excessive play? I am, however, unsure how to check the wastegate valve for leaks without removing the turbocharger .

Best regards, Rainer.

Deleted upon request.

Hello Gustav:

I believe the entire problem stems from the fact that the term "idle speed" is not sufficient to adequately describe the operating condition.
To provide a clear definition, in addition to the speed, information about the motor load is also needed.

If, for example, data logging was performed under partial or full load starting from idle speed, then the specified target boost pressure value could be realistic.
In a true idle state, meaning with zero load and at the idle speed, the target pressure should be close to atmospheric pressure, differing by only a few millibars.

Regards,
Alex.

At idle speed, you can also give it full throttle .

Hello,

"No, no, by 'idle speed' I really mean the engine running on its own, without me using the accelerator pedal. VAG-COM then shows a duty cycle of 5.1% for the turbocharger control valve in the last column; I logged measurement block 011. According to the target values here from this forum, this duty cycle should be between 30 and 80%, but mine only reaches a maximum of 40."

"@Alex: Is it correct that the target pressure should only be a few millibars above atmospheric pressure at idle speed? Because even without pressing the accelerator, the exhaust gases still drive the turbo."

Regards,
Thomas.

Please excuse the delay in responses, or my inability to immediately address your suggestions for troubleshooting, as I am currently quite pressed for time .

Hello,

At idle speed and with no throttle applied, as Gustav already mentioned, no significant pressure is generated. In my AHF system, for example, the pressure is about 0.05 bar when the engine is warm.

If your target value at idle is that high, it could be that your ambient pressure sensor in the control unit is providing incorrect readings, and the control unit software is not able to handle this?
Take a look at the ambient pressure value in the control unit.
Measurement block 10, value 2.

Best regards, Rainer.

@Rainer (thank you also for your quick and helpful responses, even for someone like me who is not an expert).

MWB 10, column 2, has a constant value of 1.027 mbar, which seems reasonable to me. Column 3 (is this the actual boost pressure?) shows a value of 1.030 mbar, but it then drops to 989 mbar, creating a vacuum in the system. Is that even possible?

Column 4 (Load in %) consistently shows a value of 0, while the value in Column 1 (Air Mass) fluctuates between 493 and 501. If, as I suspect, the value in Column 1 actually represents the air mass, it is significantly higher than the target or actual values, which according to MWB 3 should be between 210 and 370. This could also explain the smoke development, because due to excessively high LMM (air mass) values, too much diesel fuel may be injected incorrectly.

Greetings.
Thomas, who also wants to thank everyone else.

@klopfer:

Your boost pressure is perfectly normal. However, the amount of fresh air I'm seeing looks suspicious and suggests a deactivated EGR system (deliberately or unintentionally?).
A too-high LMM value will never cause smoke; rather, it tends to prevent it.
Instead of injecting more fuel, the system will only inject a maximum amount that is allowed by the torque limit.

If you didn't intentionally deactivate your EGR valve, then the measurement might indicate a sticking EGR valve. This could cause it to occasionally get stuck in the open position, which at higher loads could create a bypass of the intake air directly into the exhaust system. In this case, increased smoke emissions would be the result, as the bypassed air is not available for fuel combustion.
Try checking the functionality of your EGR valve or temporarily disable it (see a relevant technical article). If this resolves your smoking issues, you'll know what the cause is.

Regards,
Alex.

Deleted upon request.