Vacuum in VTG Can: Troubleshooting & Solutions

Hello,

It's about an Automotive Fuse Network (AFN) in a Passat 3B.
"Can anyone tell me what the vacuum pressure should be at the VTG (variable turbine geometry) control unit when idling? I'm not getting full boost pressure until around 2100 RPM, whereas it used to be around 1700-1800 RPM. The VTG should be completely closed at idle, but I'm only measuring about -0.45 bar at the control unit. At the inlet of the N75 valve, I'm seeing -0.95 bar, and all the lines are leak-free. Unfortunately, I don't have a VAG-COM diagnostic tool yet. Can I assume that the N75 valve is faulty?"

Regards,
Günter.

Without speaking ill of VAGCOM.
Because with MIT, you can see what the TCU (Transmission Control Unit) is planning to do (in the PWM signal) and what it is achieving (in the boost pressure).

Measurement block 004, in the basic settings mode, allows the N75 valve to jump from 0 mbar to "full vacuum."

Please see the latest ZIP file I sent.
..search for "ASV assessment" or "ASV evaluation"

m;

gpo wrote:

Hello, It's about an Automotive Fuse Network (AFN) in a Passat 3B. "Can anyone tell me what the vacuum pressure should be at the VTG (variable turbine geometry) control unit when idling? I'm not getting full boost pressure until around 2100 RPM, whereas it used to be around 1700-1800 RPM. The VTG should be completely closed at idle, but I'm only measuring about -0.45 bar at the control unit. At the inlet of the N75 valve, I'm seeing -0.95 bar, and all the lines are leak-free. Unfortunately, I don't have a VAG-COM diagnostic tool yet. Can I assume that the N75 valve is faulty?"

Hi,
"I measured exactly those values as well. I'm not sure if they are correct, but my turbo is acting up (although others are working fine)."

Regards,
Jens.

Hello,

I've read here several times that the VTG (Variable Turbine Geometry) is completely closed at idle, but that's not the case with mine. When I give it some gas, it opens even further. That's why I suspect a delayed turbocharger boost. As a test, I connected the VTG actuator directly to the vacuum source, and it immediately closes completely. With that setup, I can build up a good amount of boost even at idle. The VTG mechanism is easy to move, and the turbocharger shaft has no noticeable radial play.

Regards,
Günter.

gpo wrote:

I have read here several times that the VTG (Variable Turbine Geometry) is completely closed at idle, but that is not the case with mine. When I give it some gas, it opens even further. That is why I suspect a delayed build-up of boost pressure. As a test, I connected the VTG actuator directly to the vacuum, and it closes completely immediately, and I can already build up a good amount of boost pressure at idle. The VTG is easy to move, and the turbine shaft has no noticeable radial play.

Hi,

Your observations are quite good.

For more information, please read our technical article on the function and settings of VTG chargers .

Hello,

I am familiar with the technical articles. Without a diagnosis, I won't be able to proceed anyway. And I don't like to randomly replace parts . I would just like to know how the AFN behaves at idle: is the VTG fully closed or only partially closed?

Best regards,
Günter.

gpo wrote:

I would just like to know how the AFN behaves at idle: Is the VTG fully open or only about half

?
I have an MSG file from a G3 AFN (021GH), and it indicates that the VTG in the LL should be fully closed – unless I've missed any hidden limitations.

Quote:

I measure the pressure using a pressure gauge, but I only get about -0.45 bar at the

can}. At the inlet of the N75, I get -0.95 bar, and all the lines are leak-free.}

In addition to Ulf's contribution:

"Some time ago, I also measured the vacuum pressure at the turbocharger using an AFN (Air Flow Needle). At idle, it was approximately -0.7 bar. With this value, the vacuum actuator was fully engaged. -0.45 bar (if that value is correct) is not enough to fully actuate the actuator. If I remember correctly, it requires a vacuum of around -0.65 bar. I cannot guarantee that the measured values are 100% accurate, as an uncalibrated measuring device was used."

Hello,

Thank you for the answers.
I measured the pressure using a pressure gauge (vacuum technology, industrial quality) with a display range from 0 bar to -1 bar. Therefore, the values should be quite accurate.
Next week, I will investigate the issue further. A friend of mine has a VAG-Com diagnostic tool.

Regards,
Günter.

Hello gpo!

Unfortunately, you haven't provided any information about the engine's mileage. It's possible that the N75 valve isn't the cause of the low vacuum, but rather that the vacuum pump is worn out.

I have a similar suspicion about our G4 (with 250,000 km on the odometer) and would like to measure the vacuum pressure at the vacuum pump at some point and compare it to the values of my A3.
I even suspect there's a dependency on oil temperature and engine speed. With cold oil, the vacuum seems quite decent at low RPMs, but this changes rapidly as the oil temperature increases (subjective impression based on the airflow). At higher RPMs, the vacuum pump also works better and generates sufficient vacuum.

Luckily, I even have some in reserve in the basement . I just don't have the time for the analysis and the possible modification.

Regards,
Alex.

Hello donalexo,

In my first post, I wrote:

Quote:

At the entrance to the N75, I measured -0.95 bar

.
The mileage is 207,000 km.

Regards,
Günter.

Did someone not mention here that you could use the AGR control valve instead of the N75 valve to test the N75?

Hello,

I just replaced the N75 valve, and now everything is working properly again. The VTG is closing completely at idle, and the boost pressure is building up faster and earlier (0.8 bar at 1500 rpm). Thank you for the tips you gave me! Now it's probably time for me to get a VAG-Com diagnostic tool.

Best regards,
Günter.