VTG Loader: Operating principle and settings (Articles)

VTG chargers have two adjustment screws that are sealed with tamper-evident paint from the factory.

To understand their function, one should first be familiar with the basic principle of VTG (see reference). (Appendix).
See also: VTG adjustment on VAG TDI engines

In idle mode, the adjustable vanes direct the exhaust gases at an angle towards the exhaust flange, providing minimal drive to the turbine, and the boost pressure build-up is minimal. The exhaust gases flow through wide passages between the guide vanes, meaning that the exhaust back pressure of the turbocharger is also minimal. This position, or the limit of adjustment, is referred to as "VTG open."

At idle and at low engine speeds, the charging chamber receives the maximum vacuum, which rotates the guide vanes into an approximately tangential position. Between them, the exhaust gases flow through nozzle-like, narrow passages and are strongly accelerated. This creates an exhaust swirl with a high rotational speed within the turbine housing, which transmits a high driving torque to the turbine. This allows significantly more boost pressure to be generated with relatively small exhaust gas volumes compared to wastegate turbochargers of comparable size, but at the expense of high exhaust backpressure. This position or setting is referred to as VTG.

Within the mid-range speed and load range, the boost pressure can be largely freely adjusted within the limits of the engine tuning (and the material's load capacity) by adjusting the guide vanes between the impeller blades.
To prevent the boost pressure from becoming too high when accelerating at full throttle, the variable turbine geometry (VTG) must generally be adjusted from the closed position starting around 1500 rpm.

Close to the closed-throttle position, the system already reacts to even slight deflections of the guide vanes with significant changes in boost pressure and exhaust back pressure. Both affect the AGR (Abgasrückführung) system, as the recirculated exhaust gas flow is initiated by the pressure difference between the exhaust manifold and the intake manifold.
Therefore, VTG loaders have a adjustment screw on the closed-end latch, which is set at the factory on the loader test stand.

If the VTG (Variable Turbine Geometry) is not closed sufficiently, the pressure build-up occurs at unnecessarily high engine speeds, which increases the sluggishness during acceleration and the turbo lag, especially noticeable when driving at full throttle.

When the variable geometry turbine (VGT) is closed too much, the passage between the turbine blades becomes too narrow, which excessively restricts the exhaust gas flow. Result: Low turbine drive results in insufficient boost pressure again, but this time with higher exhaust back pressure, meaning that, especially when accelerating, the engine is essentially suffocated by its own exhaust gases.
When accelerating hard and the engine is running at low RPM, the variable turbine geometry (VTG) retracts from its closed position. The guide vanes then reach the area of the normal closed position with only a minimal rotation. This causes a significant drop in exhaust back pressure, while the boost pressure increases rapidly. As a result, the torque jumps almost instantaneously from "too low" to the normal values, even in high gears, instead of increasing gradually as usual. This extremely disharmonious torque curve is the most noticeable difference compared to a VTG (variable transmission gear) that is not closed sufficiently.
To make these symptoms even more puzzling, the engine can exhibit seemingly random behavior that is ultimately influenced by weather conditions and altitude: The higher the ambient pressure, the longer the variable turbine geometry (VTG) remains closed when the magnetic valve starts to change its duty cycle (too far), and the longer the engine struggles at low RPMs until the VTG begins to open, resulting in a surge of torque. With a slight misadjustment, an engine might behave almost normally at low-pressure altitudes in low mountain ranges, while on the coast, even in good weather, it might not produce enough power to pull a fish from a plate.

In the mid to high RPM range, engines with adjustable VTG (Variable Turbine Geometry) can behave completely normally, as the closed-loop control has no effect in this range.

Within the range of incorrect settings for the closed throttle position, there is a relatively narrow band of good drivability at low RPM, which is where the turbocharger is optimally adjusted by the manufacturer.
The VTG's open-type coupling is less critical and therefore does not have any adjustment options.


With the VTG rod length, the regulatory offset of the boost pressure control loop is adjusted. Therefore, any change in the length of the rods alters the tuning of the boost pressure control system.
If the length of the connecting rod is shortened to achieve higher boost pressure, this approach may or may not be effective, depending on the programmed control strategy of the engine control unit (ECU). However, it can increase the "overboost" effect when the throttle is fully opened, which puts more stress on the turbocharger and head gasket.
If the VTG (Variable Turbine Geometry) cannot be fully opened due to a shortened shaft, and the compressor housing is already at its limit, the compressor speeds in the Pmax range can increase so much, especially in tuned engines, that it can destroy the turbocharger!
If you lengthen the rod, the over-travel will decrease, but the engine usually responds more sluggishly to throttle input because the boost pressure builds up more slowly.
In addition, if the rod is misaligned, it can generate periodic oscillations, which manifest on an LDA as a constantly increasing and decreasing pressure, even when the vehicle is traveling at a constant speed and the accelerator pedal is not being moved.


The discerning reader should now have realized that there is essentially no reason to adjust the factory settings of a VTG turbocharger, unless there is suspicion of improper calibration, either from the factory (which would be an absolute exception), or due to modifications made by a previous owner.

"Any twisting of the closed-loop coupling and VTG (Variable Turbine Geometry) rod changes, among other things, the exhaust back pressure in the partial load range, and thus disrupts the EGR (Exhaust Gas Recirculation) control, in addition to the turbocharger boost pressure control."
Whether, during operation, the overall balance between increased exhaust backpressure and increased boost pressure tends to increase or decrease the EGR rate is practically impossible to predict.
As long as the engine is idling and at low RPM with minimal engine load, and no boost pressure is yet being built, a more closed VTG (Variable Turbine Geometry) with a corresponding increase in exhaust back pressure increases the EGR (Exhaust Gas Recirculation) rate, which can be observed, for example, in the default setting of parameter MWB 3 (or the dropdown selection for newer engines) after adjusting the stop.
The amount of excess emissions that may remain after the correction applied by the MSG (Motor Vehicle Supervisory Authority) cannot be predicted.
Overall, any change to the charging settings also carries the risk that the intake passages behind the EGR valve may become clogged more quickly/severely (see EGR valve)!


If there is suspicion of incorrect adjustment of the turbocharger, VCDS can be used in the basic settings mode of control module 11 (or the dropdown selection for newer engines) to quickly check whether the variable turbine geometry (VTG) is functioning and whether the closed-throttle setting is correct.
During this process, the VTG (Valve Travel Gauge) switches approximately every 10 seconds between its readings, and the setpoint display shows "ON" (closed) or "OFF" (open).
More recent engine control units (ECUs) automatically increase the idle speed to approximately 1400 rpm and compensate for speed fluctuations (caused by varying exhaust back pressures during switching of the variable turbine geometry).
The exhaust gas volume flow is sufficient to generate approximately 100 to 200 mbar of boost pressure in systems that are functioning correctly and with a warm engine. This pressure difference, between the minimum and maximum values, can be read on the VCDS display (or in log files). This pressure difference is the actual criterion for evaluation.
For this test, the engine must naturally produce the normal exhaust flow, meaning it must be fully functional, with correct valve timing, pump settings, etc.

If the MSG (Motor Steuergerät - Engine Control Unit) does not increase the idle speed in the basic setting of the MWB 11 (or the dropdown selection for newer engines), the engine can be brought to approximately 1400 rpm by giving it a little gas. However, this speed will fluctuate significantly due to the varying exhaust back pressure.
When the engine speed drops with the variable geometry turbocharger (VTG) closed, the pressure difference across the turbocharger often falls below 100 mbar, even with correctly adjusted turbochargers. This is unless the engine speed is manually adjusted back to approximately 1400 rpm after each VTG switch using the accelerator pedal, which, however, usually doesn't work as well as the automatic control provided by the engine control unit (ECU).
Therefore, pressure differences outside the specified range of approximately 100-200 mbar can also be achieved without the turbocharger necessarily being incorrectly adjusted.
Furthermore, regarding potential... Adjust the charger settings according to the following instructions.


Since the VTG (Variable Transmission Gear) is initially set to the maximum load force of the pressure vessel, mechanical clamping mechanisms can be bypassed. However, this can lead to poor control behavior and excessive overshooting during operation. That is to say, a basic setting result within the target range does not automatically mean that the VTG (Variable Transmission Gear) is working completely normally!
Mechanical clamps can be securely fastened using a large syringe (60 ml or more, possibly obtainable from a veterinarian) by slowly drawing air out of the charging chamber or releasing it. If the VTG mechanism follows the movement of the syringe plunger unevenly or jerkily, then the VTG or the chamber is jammed.

If the boost pressure difference at the default setting deviates significantly from the aforementioned 100-200 mbar at 1400 rpm and with a warm engine, then either the VTG (Variable Turbine Geometry) travel is too small or too large, or the boost pressure is escaping due to leaks, or the turbine wheel of the turbocharger is damaged or too stiff.

For further review or When performing a repair, it is advisable to follow this sequence.
1. Inspection of the VTG rod adjustment
Here's the translation:

"General rule: When the engine is running, the pressure cylinder should push the VTG lever to the open position with a slight force (approximately 5-10 N), meaning the rod and joints are fixed by the pressure and should not feel loose." If the VTG rod is pressed with the aforementioned 5-10 N force towards the container, it must disengage from its stop and move against an increasing spring force.
If not: Adjust the rod length accordingly. A (later) fine-tuning can be performed based on the height of the overshoots or the deviation from the target value, as indicated in the VCDS logs (ideally checked at full throttle in higher gears).

2. Check the operating mode of the VTG's working hub in the basic setting.
Set point = approximately 1 cm. At the closed position, the VTG (valve travel guide) should be firmly fixed by the force of the can.
If not, measure the strongest vacuum at the VTG (Variable Turbine Geometry) canister, for example, using a vacuum LDA (Laser Doppler Anemometry) connected via a T-piece: the minimum value should be 600 mbar of vacuum. Most engines produce the maximum vacuum at the VTG even at idle, but some only do so when accelerating at low RPM. If the engine does not reach 600 mbar at idle, a short test drive should be performed, including at least one brief acceleration from idle.

Immediately after the outlet of the vacuum pump, a vacuum pressure of at least 800 mbar should be present when the engine is running.

If there is a lower-than-expected vacuum pressure in the pressure reservoir or at the pump: Check for and repair leaks in the pneumatic system (including the brake booster!), and identify and eliminate any restrictions, if necessary. Replace the pump.
If there is insufficient vacuum pressure at the charging valve, the LD solenoid valve is also suspect! For a more detailed inspection, the hose to the vacuum-operated LDA should be temporarily routed into the interior, and the vacuum pressure should be checked during a full-throttle acceleration in 3rd or 4th gear. Observe an engine speed range of approximately 1100 to 4000 rpm while driving.
Here's the translation:

"General rule: Initially, the vacuum pressure rises to its maximum, then drops significantly when the turbocharger kicks in (depending on the engine speed, between approximately 1500 and 2000 rpm), and continues to decrease more slowly as the engine speed increases."
If the vacuum behaves significantly differently, then the LD solenoid valve is likely defective – unless the MWB11 log shows a corresponding pattern in the duty cycle during the relevant time period, for example, due to LD setpoint-actual deviations caused by a clogged VTG or similar.

If the VTG hub appears too small despite sufficient vacuum and a normal vacuum behavior during acceleration, the VTG mechanism, including the pressure sensor, should be checked for smooth operation (see above), possibly. Remove the jammed part or replace the canister – if you can find one, because repair shops generally no longer sell individual parts.
In tuned engines, parts of the variable turbine geometry (VTG) can be warped due to excessively high exhaust temperatures, which can make it impossible to remove the clamping mechanism. Then the only solution is a new turbocharger, and better tuning so that the new one doesn't get damaged as well...


3. If the variable geometry turbocharger (VTG) is fully extended at both ends and a pressure difference of less than 100 mbar is not achieved at approximately 1400 rpm (despite a functioning turbine wheel, a tight charge air path, etc.), then the closed-position setting is likely misadjusted.
If the pressure exceeds 200 mbar significantly, the VTG (variable geometry turbine) will close too tightly. The cause could be due to an incorrect adjustment of the stop, or the pressure exerted by the dosing mechanism over time may have left an imprint of the screw on the VTG lever, thereby shifting the stop.

VAG workshops are instructed by the manufacturer to (at the latest) concede defeat at this point and replace the charger. Apparently, it is considered impossible, or at least too prone to errors, to implement a practical new procedure for re-calibration of closed-loop systems without a dedicated testing stand and factory training, to the point where those involved are unwilling to accept the potential consequences.
The author cannot assess to what extent legal reasons (emission values) related to the impact on the EGR function may be involved in this case.

However, since such considerations are not entirely unfounded, here's a reminder:
Adjusting the closed-loop setting should only be attempted after all other potential sources of error have been definitively ruled out, due to the sensitivity of this configuration.
If the VTG (Variable Turbine Geometry) is inadvertently adjusted too tightly, not only can the exhaust back pressure increase dramatically (potentially pushing the EGR valve against its spring force), but depending on the design and wear of the VTG mechanism, parts may become detached. Subsequently, the guide vanes could rotate uncontrollably and potentially come into contact with the turbine wheel. In the workshop, the shavings will fly everywhere, and the loader will be completely ruined!
Therefore, the first adjustment of the closed-loop system should always be in the direction of a smaller overall VTG (Vertical Travel Gauge), meaning you should turn the screw approximately 0.5 to 1 revolution inwards!

To ensure that changes to the trigger adjustment are clearly visible in the default log, it is recommended to always adjust the screw by approximately 0.5 to 1 rotation, and to make smaller adjustments only when necessary for final fine-tuning.

Before making any adjustments to the settings, it is recommended to log approximately 3 switching cycles in the default configuration and analyze the actual state regarding pressure differences and speed fluctuations during the switching process.
The goal of the DIY valve adjustment is to achieve a pressure difference of approximately 100-150 mbar with a cold engine, while minimizing fluctuations in the switching RPM.
With a warm engine, the pressure difference is approximately 30 mbar higher.
Tip: Start VCDS while the engine is still off, and only run the engine for short periods to prevent the turbocharger from overheating before proceeding with further work.

If the initial tightening of the adjustment screw results in lower pressure differences, then the VTG (Variable Turbine Geometry) is not closed sufficiently -> the screw must be loosened again, but under no circumstances should it be loosened beyond the maximum pressure difference (which, in the case of intact turbines, should be 200 mbar or higher, and thus already exceeds the target value of 100 - 150 mbar)!
If the initial adjustment of the control screw results in larger pressure differences (and smaller speed fluctuations during VTG switching), then the engine previously had to struggle with excessive exhaust back pressure and a lack of boost pressure when accelerating from a low RPM, as mentioned above. Then, the screw must be turned all the way in until the target value of approximately 100-150 mbar is reached (beyond the maximum pressure difference!).

To prevent the bolt from loosening during operation, the lock nut should be tightened at least slightly before starting the engine, and at the end of the adjustment, it should be tightened with the full torque specified (according to the torque wrench used, as there are no official specifications for this outside of the manufacturing plants).

Those who want to monitor the effects of modified charging settings on the AGR (Adaptive Grille) rate should compare corresponding before-and-after logs.
1. "Baseline measurement MWB 3 at idle speed. Here, the (low) air mass value with the EGR active is interesting, as it shows the effects without any corrective control intervention."
2. Warm engine idling and operation in the lower load range (with as similar driving profile as possible). Here, the deviation between the target air mass and the actual air mass is interesting. If, after adjustment, the actual value is still, on average, below the target value compared to before, then the actual EGR rate has likely increased. Correspondingly, increased deposits of soot-oil sludge in the intake manifold may potentially be avoided through an adaptation of the AGR system.


"Whoever now (regardless of any potential..." Problems with the AGR (Abgasrückführung) can often lead to disappointment. Drivers who anticipate increased power by prematurely engaging their VTG (variable turbine geometry) to gain more boost at low RPMs will usually be disappointed – unless their turbocharger is completely misadjusted.
The author tested a KKK turbocharger on an ASZ engine, initially set to the factory setting of 120 mbar, and adjusted it to approximately 190 mbar of pressure difference while the engine was warm. However, after the adjustment, the car's performance in the lower RPM range was practically the same (or equally poor) as before.
The comparison of the AGR (Abgasrückführung) rates in the basic setting of the engine control unit (ECU) 3 (i.e., without any control intervention) was even more evident: due to the tighter VTG (variable turbine geometry), the amount of exhaust gas recirculated increased by approximately 10%.
While the deviation from the regulation remained unchanged during both idle and driving conditions, this result cannot be automatically applied to other engines.