Boost pressure control exceeded 3.0 bar, P0299 code / rough running (Solution)

Hello,

I hope I'm in the right place and haven't missed anything in my search. Our A6, model year 2012, with a 313 HP BiTDI engine, has recently started having a problem: when it's cold, meaning only when it's really cold and has been sitting for 12 hours or more, it almost 90% of the time experiences an issue within the first 1 km or 3 minutes of driving where the check light (glow plug symbol) illuminates and the engine goes into limp mode, with no power. Turning the engine off and on again completely resolves the issue, and it only reappears as described. After about a week, the following issues arose: While the engine runs smoothly at idle and starts without problems when cold, it stutters and hesitates during the first few meters. Alternatively, even if it runs smoothly at idle with the transmission in Park or Neutral, it struggles to increase the RPM when the accelerator is pressed. It jerks and hesitates severely until it reaches around 2000 RPM (which can take about 5 seconds). Once it's above that, it runs smoothly at the limited 2500 RPM like a well-oiled machine. "This only happens when it's completely cold; after 1-2 minutes, it slowly goes away, and then everything is fine, with no problems whatsoever."

The same error always occurs.

The vehicle needed to have a diesel update performed at approximately 202,000 km, which might be relevant.

Thank you for your help and ideas.

Best regards,

Thomas.

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

"1 error found:"
5171 - Charge pressure control.
P0299 00 [237] - Limit exceeded.
Warning light ON - confirmed - checked since last reset.
Environmental conditions:
Error status: 00000001
Error priority: 2
Error frequency: 1.
Mileage: 206,612 km
Date: 2020.08.18
Time: 07:40:11
Motor speed: 1759.00 rpm.
Normalized load value: 87.1%
Vehicle speed: 71 km/h.
Coolant temperature: 56 °C.
Intake air temperature: 26 °C.
Ambient air pressure: 960 mbar.
Voltage at terminal 30: 13,600 V.
Lost count after OBD: 40
Boost pressure controller: Control signal: 91.50%.
Boost pressure sensor: Feedback: 91.09%.
Outside temperature: 19.0 °C
Boost pressure: Target value: 2122 hPa.
Gas pedal travel: 37.44%.
Air mass: Actual value (mg/stroke): 769.2 mg/stroke.
Boost pressure actual value: 1706 hPa.
Readiness: 1 1 0 0 0

Hello,

The first thing I would check is whether the intercooler system is airtight. "Leaking" means insufficient or no boost pressure.

Best regards, Rainer.

Hello,

Thank you. I did a pressure test to make sure everything is sealed, although I couldn't imagine it leaking because the error only occurs during the first 3 minutes and only when the system is completely cold; after that, the error never appears.
The diagram in VCDS showing the target vs. actual boost pressure looked okay to me...

Best regards,
Thomas

Hi,

Please upload the raw VCDS log files; the image is not very informative. I hope you're familiar with KDataScope...

If the error only occurs when the engine is cold: Is the software in the engine control unit original, with no chip tuning or power box?

Best regards, Rainer.

"With 206,000 km on the clock, it's not surprising that some of the charging system components might start to fail... at least one of the many adjustment mechanisms or actuators/elements. I once had one in my hands - it was absolutely horrifying - completely different from my V10 TDI - simply terrible!" SG Manfred.

What does the error memory contain?

Evening.

The early BiTDI engines often had issues with the injectors.
Back in the days when we had a car dealership, we changed them more often.
If the engine is only running on 5 cylinders, it's also possible that this is preventing the turbocharger from generating boost.
Again, the turbocharger replacement was also not uncommon .

Best regards,

Hello,

"The error was very sporadic, occurring once a week in the mornings, but recently it has become increasingly frequent. Now, if you monitor the data, you can even observe a difference between the target and actual boost pressure. However, if you press the accelerator 20 times, it only achieves the correct boost pressure 19 times; if it doesn't reach the target pressure, it simply doesn't accelerate." Try and pressure!
There is no deviation between the setpoint and the actual value from the VTG adjuster.
Today, however, the pressure is so low that it's almost not being reached, even if the "large" loader is supposed to be fully engaged. Initially, nothing was visible, but now there's a spot on the exhaust manifold in area 3. "Cylinder... and it smells different over there, on that side..."

Thank you for your help. I will let you know if there are any new developments.

Hi.

Quote:

If I press the gas pedal 20 times, it will have 19 times the pressure, meaning that if it doesn't reach that pressure, just release the gas, 2. Try and pressure! There is no deviation between the setpoint and the actual value from the VTG adjuster.

Does the error memory only ever contain your entry from above?

The boost pressure regulator may not always function correctly. However, in the event of cable problems or internal issues within the regulator, a difference in the regulated pressure should be visible. Or an error entry regarding this.

Mechanically, is the adjuster or control rod in good working order and is nothing loose?

Theoretically, a faulty EGR valve could also prevent the build-up of boost pressure, but in that case, the same applies as with a faulty boost pressure regulator...

Best regards, Rainer.

Yes, that's right. The error log always only shows the error as described in the previous post.

The VTG controller operates normally, both when cold and when warm, without any noticeable issues.
I compared it to another identical car that had only 40,000 km, and there was no noticeable difference in how it felt or in its behavior during the actuator diagnostics.

If I run the VCDS diagram in the background, along with...
Boost pressure: Target value/Actual value --> Deviation as described.
Fuel pressure: Target value/Actual value --> No deviation.
Boost pressure setpoint/actual value --> no deviation.

We checked the intake system again today. It's sealed.
Vacuum system checked --> Leak-tight.

Guided diagnosis at the doctor's office --> no findings.

A technical inquiry has been sent to Audi -- let's see what ideas they come up with.

I want to take a look at the bypass valve for the supercharger, which is located parallel to the "small" turbocharger, just to make sure it's not leaking and just circulating air. Occasionally, there was an issue where, around 3500 RPM, it felt like the power was momentarily reduced. It's possible that the bypass valve was sticking, which would happen if the "large" turbocharger was providing more boost than the "small" one, and the valve was supposed to open to relieve the pressure. We'll have to investigate.

Quote:

I want to take a look at the compressor bypass valve, which is located parallel to the "small" turbocharger, just to make sure it's not leaking and that it's not just pumping air in a loop.

Hello....

I still suspect it's related to a "hot" connection or short circuit caused by some kind of actuator or VTG (Variable Transmission Gear).

If you arrive late, you may need to treat the affected areas (attachment points) with "VW Hot Mounting Paste G052112A3" or a similar lubricant.

If there are any flexible hoses there (I can't quite remember now), I would carefully check them – they often tear somewhere, even just a little bit – and then only start leaking at a certain temperature and boost pressure.

You could also try attaching a GoPro or similar camera and letting it record while it's running... maybe you'll be able to see something. I had a similar problem (VTG stuck) with my Golf, and I was able to fix it with an additional locking spring -- see the video attached. I, however, doubt whether this is a viable solution - although it might be possible if you have a vacuum-controlled setpoint.

See video (starting around the 60-second mark!).

Best regards.
Manfred.

Now that the error has been found, I would like to inform you about it.

The response to the technical inquiry from Audi was...
Carefully check the vacuum system; the vacuum pressure should be higher than what is indicated in the diagnostic guide, specifically at least 850 mbar!

The vacuum pressure was checked a third time, and this time it dropped rapidly to only 300 mbar. The cause was quickly identified: the diaphragm in the vacuum sensor for the exhaust cooler was defective.
The reason the error occurred sporadically was also relatively easy to reproduce. If the vacuum was already sufficiently high, and the vacuum sensor was then activated, it would switch completely and maintain the vacuum. However, if the vacuum sensor was activated before the vacuum had fully built up, the pressure loss was so high that the vacuum would not drop below the 300 mbar that I measured. And what does all of this have to do with boost pressure?
"In the bi-turbo system, there's a turbine bypass valve that, in addition to the VTG (Variable Turbine Geometry) adjustment, acts like a wastegate for the small high-pressure turbocharger. At 300 mBar, it may not seal tightly enough, and the small high-pressure turbocharger, therefore, may not build up enough boost pressure. In my case, this isn't always the issue. The vacuum actuator for this turbine bypass valve has a potentiometer, meaning there's a setpoint/actual value in the engine control unit. I took the trouble to examine the values in case of a fault. The setpoint is usually 101%, while the actual value drops from 100% to approximately 98% in case of a fault. This is a relatively small deviation compared to the setpoint/actual value of the boost pressure, before the engine control unit..." The turbine bypass valve detected a deviation from the expected boost pressure and initiated emergency mode.

To further diagnose the issue, I disconnected the vacuum hose from the exhaust cooler. This initially only creates a bypass for the exhaust cooling system -> the error is gone!
Therefore, it was possible to determine exactly what needed to be repaired.

Thank you again for your help!

Quote:

The vacuum was checked a 3rd time, and this time it dropped quickly to only 300 mBar. The cause was quickly found: the membrane of the vacuum sensor from the exhaust cooler was defective!

Great! For the sake of completeness for other readers: There have also been cases where the vacuum sensor of the turbocharger was leaky in a certain position, or the vacuum sensor of the throttle valve was leaky (defective).

Best regards, Rainer.