Turbocharger Bypass (TCBY) Valve
WARNING: This page does not describe the selected car, but rather 32 other vehicles, including the 2011 Mercury Milan, 2011 Mercury Mariner, 2011 Mercury Grand Marquis, 2011 Lincoln Town Car, and 2011 Lincoln Navigator L. However, it is still accessible from the selected car via links, so may be relevant.
The TCBY valve(s) prevent backflow through the turbochargers when the throttle is rapidly closed to avoid undesirable noise. The high pressure downstream of the turbocharger is vented back to the intake air stream when the valve is open reducing pressure in the system.
Two types of bypass valves are currently being used. The first is a solenoid controlled valve located in a crossover tube between the turbocharger intake side and the pressurized output to the charge air cooler (CAC). The second type of bypass valve is an electropneumatically controlled system consisting of a vacuum source, control solenoid, tubing and a vacuum controlled bypass valve that is integrated into the turbocharger housing.