Boost pressure is a crucial parameter for evaluating turbocharged engine performance. Using VCDS (Vag-Com Diagnostic System), you can monitor and log this data using specific measuring blocks. This article explains how to use VCDS to measure boost in both gasoline TSI and diesel TDI engines, focusing on the difference between requested and actual boost.
Understanding the discrepancy between requested and actual boost can help diagnose potential issues within the turbocharging system. This involves logging specific measuring blocks within VCDS and analyzing the resulting data. For TSI engines, boost is typically found in Group 115, while for TDI engines, it’s in Group 11.
To accurately assess boost performance, it’s essential to log relevant data alongside boost readings. For TDI engines, simultaneously logging Group 03 (MAF and EGR values) or Group 10 (MAF and gas pedal position) provides valuable context. In TSI engines, logging a group between 110 and 119, indicating throttle valve angle or gas pedal position (e.g., Group 117), is recommended. If the TSI engine is equipped with an EGR valve, logging Group 76 is also beneficial.
Performing a proper boost log requires a specific driving procedure. On an open road, with the car in 3rd gear, start logging the chosen groups at a low RPM (around 1000-1500 RPM). Then, fully depress the accelerator pedal and continue logging until reaching 4000-5000 RPM, depending on the fuel type (gasoline or diesel). The goal is to capture data during a period of full load and increasing RPM, ideally between 2000 and 4000 RPM.
Plotting the logged data with requested and actual boost on the Y-axis and RPM on the X-axis provides a clear visual representation of boost behavior. Including MAF and EGR values on the same plot can further aid in identifying any anomalies. These values should behave logically under full load: MAF should be at its maximum, and EGR should be at its minimum (closed).
A common observation in gasoline TSI engines is a difference between requested and actual boost, especially at partial throttle. This discrepancy is primarily due to the MAP (Manifold Absolute Pressure) sensor being located before the throttle body. In contrast, TDI engines lack a throttle body, resulting in the MAP sensor more accurately reflecting intake pressure and requested boost even at partial load.
Despite these differences, accurate boost testing requires a wide-open throttle (full load) condition in both TSI and TDI engines. The resulting data should be analyzed over RPM rather than time to provide a meaningful representation of boost performance across the engine’s operating range. Analyzing the relationship between requested and actual boost can reveal potential issues such as boost leaks, faulty turbochargers, or problems with wastegate control.
