Common Rail Fuel Pressure Regulator ,Control Valve.
Fitting Position: High Pressure Pump (low pressure side).

INFO:
The fuel pressure control valve comprises a fuel-cooled solenoid valve.
The valve opening is varied by its solenoid coil being pulse width modulated at a frequency of 1 KHz.
When the pressure control valve is not activated, its internal spring maintains
a fuel pressure of about 100 Bar. When the valve is activated,
the force of the electromagnet aids the spring, reducing the opening of the valve
and so increasing fuel pressure. The fuel pressure control valve also acts as
a mechanical pressure damper, smoothing the high frequency pressure pulses
emanating from the radial piston pump when less than three pistons are activated.

This part is called by many names including:
MPROP
FCA
Injection Pressure Regulator
Fuel Pressure Regulator
Fuel Control Actuator
Measuring Unit
Fuel Measurement Solenoid Valve .