i know the fuel goes in and there is a out and there is a vacuum source.. but what does it do to up the pressure..?

 

The boost presses on a rubber diaphragm. The diaphragm pushes on the calibration disk (the bigger the disk the bigger the push). The disk rides on a metal plunger that begins to close off flow through the unit as its pushed down.

At vacuum the diaphragm is lifted off the disk and plunger allowing all the flow through. At boost the pressure on the diaphragm restricts the flow through based on how big the disk is.

 

To continue this archived topic a little....................

So when you open the FMU the metal plunger is not spring loaded, thus don't freely move with the fmu apart?