Originally Posted by syljua

The line is there to regulate the fuel pressure in the rail
so that the pressure is at a constant level above pressure in the intake
(so that a set opening time of the injector will give the same
amount of fuel regardless of intake pressure). If the diaphragm goes
south, yes the fuel will be drawn to the intake, but thats secondary.

I made a video on checking the FPR last summer;
http://www.pvv.org/~syljua/merc/FuelPressureME.mov

This is for a M119, and I see now, that maybe the M103 FPR functions
differently?

br,
syljua