Re: Add-on device on fuel distributor (FD)
To bring the car into the US the EPA required that the fuel metering system be equipped with a feedback device that would maintain the mixture at stochiometric. In short, O2 sensing to protect the catalytic converter.
The valve is known as a "frequency" valve, and is intended to adjust the control pressure in the FD independently of the control pressure regulator (aka WUR). The frequency valve is operated by a control unit that receives a signal from the O2 sensor.
The internal fuel pressure regulator in the FD is not affected by the frequency valve.