VTG Issues: Emergency mode, suction tube pressure regulation difference, etc. (Fachartikel)

Fluctuating acceleration or intermittent acceleration, such as SDI instead of TDI; solenoid valve for regulating the charging pressure and vacuum hoses are definitely okay, but the charging pressure regulation is not working correctly...

First, a visual inspection of the VTG function should be performed: Immediately after the motor starts (by an assistant), if the VTG is functioning correctly, the threaded rod will be pulled approximately 1 cm towards the membrane housing at idle speed and will remain there.

During short bursts of gas, the rod will move slightly back and forth.

Does the mechanism behave differently, by disconnecting the hose from the charging unit and using a large syringe (min. 50 ml) to suck on the unit?

The threaded rod should normally be able to be moved delicately into any position within the total tube of approximately 1 cm using a syringe.

However, if the aforementioned visual inspection during motor start does not or only shows small movements, the VTG often gets stuck in the resting or maximum position during the syringe test, and/or it jumps somewhere in between.

Common causes include excessive soot buildup in the VTG; however, frequent, short-distance operation with minimal engine load and, consequently, little lubrication due to soot can also cause condensation to form in the VTG mechanics from the exhaust gases, leading to corrosion.

Sometimes, however, the problem lies with a faulty membrane chamber.
To check this, the chamber should be removed and operated independently with the doctor's syringe.
The maximum stroke of its threaded rod is approximately 1.5 cm, and the rod should also be able to move smoothly with the syringe.
If the chamber is defective, the only option is practically to replace it. In this case, it may also be worthwhile to switch to a larger loading chamber (see below).

To free VTGs that are stuck due to soot or corrosion, the VTG must be moved repeatedly from stop to stop until it is sufficiently free.

Possible methods:

1. Unscrew the housing and operate the threaded rod at least 100 times by hand, then reattach the housing and adjust the threaded rod to its original length (very tedious, should be done after a cold start to avoid burning your fingers).

2. Using VCDS, access the VTG (Variable Turbine Geometry) control block and perform the basic setting procedure. This will alternately drive the VTG to its minimum and maximum positions. However, this only works during engine idle and with minimal throttle. If you attempt to force the VTG to its maximum position with more throttle or while driving, the procedure will be aborted and the system will revert to normal operation.

3. Disconnect the solenoid for the fuel pressure and, using temporary wiring, activate and deactivate the solenoid in quick succession with the car's voltage. Check if the threaded rod moves approximately 1 cm.

4. Same as No. 3, but connect the solenoid to the connector of a turn signal bulb and activate the hazard lights.

5. Build your own electronic device that activates the valve at approximately 1 Hz and can also be activated under load during operation, with the stronger exhaust stream blowing away dissolved soot particles more effectively. This is only recommended if the pressure can be monitored with a boost pressure gauge to ensure that it does not rise too high (very complex).

Unfortunately, the results often only last for a few days or weeks. Sometimes, a short burst of full throttle on the highway can also help.

Further information on VTG adjustment, its inspection, and function can be found in the technical article VTG Adjustment for VAG TDI Engines.

In case of malfunctions, also consider the VTG Vacuum Control or the electrical control of the VTG.

The more permanent, but more expensive solution:

Replace the small charging case (approximately 5.5 cm outer diameter) with a larger one, as it is standardly fitted in newer TDIs.

The AFN standard case 433483-1 can be easily replaced with the larger case (approximately 7.5 cm) No. 434855-3. In this case, the larger heat shield No. R7026940001 should also be installed.

You can get the parts (or were able to get them earlier) e.g. at Schlütter Turbolader for around 120 Euros -> www.turbolader.com
Update:
Unfortunately, individual Garrett turbocharger housings are no longer available.
Other options: salvage defective turbochargers, inquire at auto salvage yards, etc.

To adjust the threaded rod with the other canister, the following is recommended:

1. The travel range of the VTG lever on the loader should be approximately 1 cm, otherwise adjust (especially if the stops also feel soft and pliable instead of firm!).
2. Adjust the rod so that the VTG lever is pressed against the "minimal LD" stop with light force (approximately 5 N) without vacuum.
3. If possible, check the required vacuum to reach the stops, e.g., using a large syringe and suction-LDA: At approximately 50 to 100 mbar, the min-stop should be left, and at approximately 600 mbar vacuum, the max-stop should be reached.
4. Fine-tuning: During test runs with LDA (if necessary, using VCDS data logs), adjust during early pressure build-up without large swings when adjusting. Test: accelerate from idle speed in 2nd or 3rd gear with full throttle.