(If you are running a manual boost controller, ignore this article as it is only relevant to those using OEM digital boost control systems)
* Track Actual VS. Requested boost to diagnose a wastegate/boost issue on 1.8T/2.0T/2.7T engines
Tools needed:
- Laptop with serial cable/USB port
- Ross-tech VCDS
- An assistant to log functions on the road!
If you are not familiar with Ross-Techs VCDS program, you will need to purchase the starter kit on there website linked above.
(Note for those with a 2.0T engine) When scanning your engine for codes, “Boost pressure regulation” and/or “Boost pressure positive deviation” codes – Look no further and replace your N249 diverter valve on the compressor housing if you have established that there are no leaks in the pressure side of the system. The N249 valve is one of the most common failures of the boost pressure system on 2.0T Audi and Volkswagen vehicles.
Step 1: Open up VCDS and establish communication with your engine computer.
Step 2: Open up “Measuring blocks” and enter in “115” for requested measuring block. Once engine is running and 115 is displayed, you will see two respective readings of exactly/very close to 990.0 mbar while your engine is idling. Any deviation or major difference between your actual and requested reading would indicate a sensor issue. Especially while your engine is idling. (millibar – metric measurement specific in this test to Audi/VW systems)
Step 3: Click the bottom right button titled “Graph” and start the process of driving around and engaging boost to view the relationship and struggle between your cars computer and its boost control system
Image may be NSFW.
Clik here to view.
Image may be NSFW.
Clik here to view.
Actual boost is below = wastegate opening prematurely, or opening too far. may be a damaged linkage
Actual boost is above = torn diaphragm or tear in the vacuum line going to wastegate
Actual boost matches specified boost pressure but turbos are over compensating/surging and spooling to quickly = smoke test/pressure test the compressor housing to the throttle body boot for leaks! The N75 valve will compensate for the difference and blow your turbos in the process of attempting to provide the engine with enough CFM!
For twin turbo setups =
Note that there is only one N75 valve (boost control solenoid) for both turbos.
No two turbos are perfectly matched. Having one valve to control both turbochargers is the factories way of elimating any performance deviations between otherwise identical vehicles with identical drivetrains. In the opinion of most engineers, it would be ideal to have a seperate boost control system for each turbo, paired with a singular solenoid to elimate any last moment deviations between the pair of turbochargers, being that they feed into only one throttle body (In the case of APB/BEL 2.7T units)
In the case of setting up your twin turbo Audi S4/A6/Allroad, I HIGHLY recommend setting wastegate opening pressures outside of the car. Please see THIS article.
Image may be NSFW.
Clik here to view.
Image may be NSFW.Clik here to view.
