When performing a VNT actuator test using VCDS, slight deviations between specified and actual values at idle are normal. For instance, a specified value of 1040 mbar and an actual value of 989 mbar at no-load idle, with the engine under slight vacuum, is not a cause for concern. This difference is insufficient to trigger closed-loop corrections, typically requiring a deviation of around 200 mbar. The ECU doesn’t consider a 51 mbar difference significant, especially at low load and RPM.
Similarly, an actual value of 1200 mbar versus a requested value of 1100 mbar in open-loop operation doesn’t indicate a problem. The ECU isn’t applying corrections in this range, as this operational region doesn’t demand extremely precise tolerances. Worrying about these minor discrepancies without further data is unnecessary.
To effectively diagnose the VNT actuator and overall turbo performance, utilize VCDS with a partner and conduct a road test. Accelerate at full throttle from approximately 1500 RPM to 4000 RPM in 3rd gear. Simultaneously, log measuring blocks group 11 in VCDS, along with group 1 for relative power reference. This data will provide crucial insights into the health of your actuator and turbo system.
The basic settings function in group 11 of VCDS primarily resets the baseline offset for diagnostics. While helpful after installing a new turbo or reflashing the ECU, it doesn’t offer detailed performance analysis like the logging method described above. A proper road test with data logging is essential for comprehensive VNT actuator and turbo assessment.
