The fuel pressure regulator is a vacuum operated diaphragm relief valve with fuel pump pressure on one side and regulator spring pressure and intake manifold vacuum on the other side. The fuel pressure regulator maintains a constant pressure differential across the injectors at all times. The pressure regulator compensates for engine load by increasing fuel pressure as the engine vacuum drops.
These valves can rupture inside and allow fuel to enter the vacuum line. This is a common cause for hard starting, rough idle for a few minutes after starting, or stall after starting.