Turbocharger Replacement: How-To Guide

Hello,

I'm still experiencing the power loss issue between 2600-3000 RPM in my 1999 ALH engine. It mostly occurs in 4th and 5th gear, under load. I've tried a lot of things (replaced the MAF sensor, blocked the EGR valve...), but nothing has worked. I went back to the VW dealer last week, and they did a test drive. Their measurements confirmed my VGA-Com readings. The boost pressure increases to about 1.4 bar, then suddenly drops to 0.5 bar, and only slowly increases again.
With a new mass airflow sensor (MAF sensor) – tested temporarily – it wasn't noticeably better. Because I have a used car warranty, he advised me to replace the entire turbocharger and the MAF sensor. What do you think about that? This week, an appraiser from the insurance company will come and inspect the car.

ABRA wrote:

My VGA-Com measurement has been confirmed. The boost pressure increases to approximately 1.4 bar, then suddenly drops to 0.5 bar, and only slowly increases again. With a new LLM (for testing purposes), it wasn't noticeably better.

Hi.

I'm not familiar with the background right now, but have you checked the entire VTG assembly, including the can, solenoid valve, and hoses?

Have you ever performed a boost pressure cross-check using an LDA (Linear Displacement Analyzer) to rule out pressure sensor issues?

Hello,

'To me, it sounds like a stuck VTG (Variable Turbine Geometry) adjustment. Try manually moving it to see if it moves freely, or let it run in idle and use VCDS (VAG-COM) to repeatedly open and close it for about half an hour. Let's see if that improves things. By the way, I can see on my boost gauge when the VTG is sluggish (i.e., difficult to move). The boost pressure fluctuates, then drops too low, and only then reaches the set maximum of currently 1.2 bar.'

'Driving at full throttle for 5 minutes might fix it, but unfortunately, that's rarely possible in real traffic conditions.'

I initially don't believe the LMM reading, because if there's no LD (likely referring to a specific sensor or component), it won't inject anything.

Hello,

Thank you for your responses. Here is some more background information:

/viewtopic.php?t=1307&highlight=

Achim.

ABRA wrote:

Here you can find more information about the background.

Hmm, so it seems the VTG is okay... just try swapping the EGR valve and the turbocharger pressure control valve as a test.
If the power loss is resolved, it's likely due to a problem with the boost pressure valve.

Hello ABRA,

In my boost pressure curve, I also experience dips, although not as severe. For example, the target pressure is 1.1 bar, with an overshoot of 1.3 bar, followed by an undershoot of 0.8 bar. And then, the boost pressure slowly creeps towards the target value .

I suspect the vacuum pump might be the issue. The vacuum system seems to be mostly airtight. Then, I completely disabled the EGR valve (removed the vacuum hose leading to the vacuum pump and connected the line to the magnetic valve for turbocharger pressure regulation directly to the vacuum pump). After that, the turbo spool-up became much better. Then, I created a restriction in the external vacuum hose that leads to the magnetic valve for turbocharger pressure (disconnected it from the air filter and screwed in a bolt). As a result, I experienced quite a violent oscillating turbo spool-up. However, by adjusting the length of the VTG lever, I was able to adjust the pressure curve as it should be: no overshooting and no undershooting anymore.

To me, this indicates insufficient vacuum pressure compared to the outside pressure, for whatever reason (leak in the vacuum system, vacuum pump malfunction).

Best regards,
Ernst.