Relief Valve
To recirculate the air at closed throttle, there is a recirculating relief valve. The valve, which is electrically controlled is similar to the bypass valve used on engine B6304T. In the event of boost in the intake manifold (boost pressure) and when the accelerator pedal is released quickly, an increasing pressure builds up that can result in noise (pulsations). The task of the relief valve is to route this pressure back to the turbocharger's inlet side.
This means that the turbo's speed is maintained and that the boost pressure is available immediately, at the same time as the noise is reduced. The air is routed back again between the mass air flow sensor and turbo compressor so as not to affect the measured mass air value. The ECM controls the valve's position on/off with 12V.