Okay so the specs for fuel pressure are 31 with the vac line on the regulator and 39 off. My question is how much of a buffer in being good is there? For instance mine is 32-34 with the vac line on and 40-43 with it off. It is not fluctuating but just tests a little different depending on the guage.. I have tried two different guages on this. Any help is appreciated! Oh yeah it's an 89. 4.0 auto

 

The spec's tell you what the best / lowest pressure for operating is. If your fuel pressure is at or a little above them your fine. From what your readings are your fuel pump and fuel pressure regulator are working.