VTG Adjustment for VAG TDI Engines (Articles)

Here, the boost pressure is no longer limited by a simple, spring-loaded flap (wastegate), but instead the runner wheel on the turbine shaft is supplied with air by adjustable guide vanes with different angles.

Function of VTG adjustment (external)

Advantages of a VTG charger include a lower engine RPM when sufficient boost pressure is available, and generally a wider operating range.

However, a significant advantage of VTG (Variable Turbine Geometry) systems compared to systems with wastegates is their increased vulnerability due to higher mechanical complexity.

How does a faulty VTG manifest itself?Therefore, it exhibits increased sensitivity to rust, oil coke, and corrosion, which are the three most common causes that should impair the function of the VTG. Excessive soot formation due to excessive tuning or engine defects, as well as oil coke, can lead to a restriction of the mechanical clearance of the adjusting vanes, which can even lead to complete blockage.

This prevents the engine management system from controlling the boost pressure, and it switches to a fail-safe mode with significantly reduced engine power.

An error in the engine control unit that points in this direction is, for example, 'Control Difference Intake Manifold Pressure'.

However, this is only an indication, and the error message could have other causes.

Therefore, a diagnostic test with a diagnostic tool is the best way to identify errors in the turbocharger pressure control system.

A correct load pressure profile at full load should approximately resemble the yellow curve in the load pressure target vs. actual comparison below.

Customers of Kaufmann Automotive GmbH have access to KDataScope, a tool for analyzing data blocks, which is included with our OBD diagnostic devices.

The entire thing can be read in the engine control unit (address 01) in data block 11 (or extended data blocks for newer engines) and can be saved to a file using VCDS.

How can I determine if a VTG is stuck?First, you need to determine whether there is actually a stuck VTG or if there is another problem with the loading pressure control or engine management.

To do this, you monitor the vacuum hose at the bottom of the vacuum chamber, connected to the VTG control. This leads to the electromagnetic converter.
(Solenoid for fuel pump pressure regulation) which is mounted on the fuel rail.

Now, you remove the suction tube, insert it into your mouth, and begin to suck forcefully. If you are unable to achieve movement using a suction pump, a large medical syringe or other suction pump can be effective.

Here, you observe the VTG adjustment lever. It must be able to move from its rest position by approximately 1 cm to the position that corresponds to "maximum charging pressure" without sticking or catching. Additionally, the hose and the vacuum canister must be in position in every position.
be dense.

Tip: Even at idle, the VTG is often set to the 'max' position in many standard engine control units. For example, with engine codes ALH, AHF, AFN, AXR, ATD, ASZ, ARL, AWX, BPX. The adjustment lever must
so, the engine should be at its idle setting, with the lever at one end position when the engine is stopped, and at the other end position when the engine is running. The total travel of the lever is approximately 1cm.

If it doesn't work, you should first check the underpressure cylinder for a broken spring or a damaged membrane. To do this, you need to remove it from the loader: Measure or mark the length of the VTG rods, loosen the nuts under the underpressure cylinder mounting plate and unscrew the VTG rod, if the underpressure cylinder is not removable from the loader housing with the screws.
Ideally, the entire underpressure cylinder should be unscrewed, rather than being turned on the VTG adjustment rod, as the adjustment is relatively sensitive.

Now, you can also simply check the ease of movement of the turbocharger adjuster lever; there should be no significant resistance.
be, if necessary, one can try to get the mechanism working again by moving it back and forth:

You can try to move the fuel pump diaphragm at least 100 times per minute between strokes, with occasional bursts of fuel in different positions of the mechanism. Continue doing this until the entire path is easily accessible even with the engine stopped. Then, reinstall the diaphragm and set the VTG lever back to its original length.

More success is promised by disassembling the charger to allow for a reasonable cleaning/inspection of the VTG. As a starting point, the above link describing the VTG functionality can be used. It is recommended to perform an initial "practice" on a faulty loader.

If the original VTG rod length is not reproducible, adjust the rod so that the mechanism is pressed against the lower stop with light to moderate force. During a test drive, test the responsiveness and, if necessary, correct the settings:
When the engine starts with unusually low power at low speeds, slightly shorten the lever. If there is excessive biting or even full-load running problems, lengthen the lever. As a general rule, the actual maximum boost pressure value during the adjustment process should not exceed the target value by more than 10%. For example, target boost pressure 1990 mbar -> maximum overshoot of 2190 mbar.
If a pressure gauge is present, adjust the lever to the desired pressure.

Tip for AHF or similar engines: Adjust the timing so that with at least 3 seconds of full throttle in 3rd/4th/5th gear at 3900 min^{-1}, the solenoid for the boost pressure control reaches an approximately 85-90% duty cycle. The duty cycle can be read out using a diagnostic tool. The maximum fuel ratio is approximately 95%, and a reserve of 5-10% should be maintained. This is because, at temperatures below 0°C, the charging pressure is more easily achieved, and the control unit must compensate for this more.

If the repair attempt fails, unfortunately, the turbocharger needs to be replaced.

How can I prevent a stuck VTG?

First, the motor should not be excessively strained, as this can also get stuck in the adjustment mechanism. Powerboxes are therefore strictly prohibited. Similarly, the repair of leaking air intake hoses/air intake coolers should not be postponed indefinitely, because the increased soot emissions that result from this not only waste fuel unnecessarily, but will also permanently damage the VTG (Variable Turbine Geometry) over time.

If this has already happened, some kilometers with full load and many load changes on the highway may allow some soot to burn away.

Furthermore, it is generally not advisable to use excessively thin engine oil. Oils with 0W... tend to leak past worn-out bearing seals and cause oil sludge.

Rust on the adjustment joint, or where the adjustment lever enters the loader housing, can be easily prevented using copper paste.

Thank you, Ulf, for finding this out:
For older TDI engines with a small adjustment housing (part number 433483-1 for engines with the designation AFN), this can be replaced with a larger one. The adjustment housing is not available separately, but can be obtained after a turbocharger replacement using the following parts:
- Larger vacuum housing 434855-3
- Matching heat shield R702694-1
With the larger vacuum housing, a greater force can now be applied to adjust the VTG.
When installing, it is essential to ensure that the previous adjustment range is maintained! A check of the boost pressure behavior (using a pressure gauge or diagnostic tool) should always be performed afterward.

If the VTG (Variable Timing Gear) is functioning, but the boost pressure curve still doesn't match?

...the next steps are to check all vacuum lines for leaks, check the functionality of the electropneumatic converter, the air mass flow meter, the fuel pump, and the control unit itself.