A diverter valve is a pressure release valve that is used to prevent unused pressurized air to back track into the turbo creating a condition known as compressor surge which will lead into turbo failure. These valves are operated by engine vacuum, when the throttle plate closes after acceleration, the diaphragm sees the pressure change then the valve opens releasing air back into the intake tract which helps with throttle response and reduces noise when the valve opens and releases pressurized air. a leaking diverter valve will lead ito power loss because of a unstable fluctuating boost level. Our diverter valves for the 135i/335i/535i are of top quality cnc machined aluminum ( the stock unit is plastic which in time with heat can get brittle) with bigger O-rings to ensure a perfect seal, pressure tested to hold 50 psi, great performance, great look, mount on stock locations, great price. waiting for a stock valve to fail is too risky , either lack of power or even worse "shaftplay" which is the imminent death of your turbochargers and if not fixed in time your engine.