When using VCDS (VAG-COM Diagnostic System) to monitor your turbocharged engine, you might notice discrepancies between specified and actual vacuum readings. For example, at idle, a specified reading of 1040 mbar might show an actual reading of 989 mbar. Similarly, under light load, an actual reading of 1200 mbar could be observed against a specified value of 1100 mbar. These minor deviations are generally normal and not a cause for concern.
The engine control unit (ECU) typically doesn’t activate closed-loop corrections for small differences, especially at low engine speeds and loads. A deviation of around 200 mbar is often required to trigger these corrections. A 51 mbar difference at idle or a 100 mbar difference under light load falls within acceptable tolerances and doesn’t indicate a problem with the turbocharger or its actuator. The ECU operates in open loop during these conditions, meaning it isn’t actively adjusting boost based on these minor variations.
To thoroughly assess the health of your turbocharger and actuator, a dynamic Vcds Vacuum Test is necessary. This involves logging data while the engine is under load. Find a safe, open road and have a passenger operate VCDS. With the car in 3rd gear, accelerate at full throttle from approximately 1500 RPM to 4000 RPM.
During this acceleration run, log measuring blocks group 11 in VCDS. This group provides crucial data on boost pressure, specified boost, and actuator position. Simultaneously log group 1 to correlate boost data with engine load and RPM. Analyzing these logs will reveal how well the turbocharger and actuator respond to changing engine demands. This dynamic test provides significantly more valuable diagnostic information than static readings at idle or low load.
The basic settings function in group 11 of VCDS allows for resetting the actuator’s baseline offset. This procedure is typically performed after installing a new turbocharger or reflashing the ECU. While useful in these specific scenarios, it doesn’t provide a comprehensive picture of turbocharger health in the same way a dynamic log does. A proper VCDS vacuum test under load is the most effective way to diagnose potential issues with your turbo system.
