ECO Valve In The Power-Steering Pump: Construction
WARNING: This page does not describe the selected car, but rather 15 other vehicles, including the 2006 BMW 530i, 2006 BMW 525i, 2006 BMW 330xi, 2006 BMW 330i, and 2006 BMW 330Ci. However, it is still accessible from the selected car via links, so may be relevant.
The power-steering pump is a vane-cell pump. The power-steering pump accommodates the ECO valve (ECO = Electronically Controlled Orifice). The ECO valve regulates the volume flow of the hydraulic fluid, thus regulating the pump's hydraulic power.