It looks to me as if the OP is getting close and there is some good advice so far, maybe a compression check and a check of the suspect injector grounds? Is this an auto or man trans?
Just for the record, there is no vacuum modulation of the FPR on most of the V6 engines (non SC) despite what the FSM says. The ones that have it were early production T 100s and possibly some early Tacos. They also had an EGR system, 99.9% sure on this last point. The intent was to increase fuel pressure on a hot restart to hasten said restart. The ECM was involved controlling a VSV that in turn would modulate manifold vacuum to the FPR. This deal was soon dropped and the little rubber hose on most Taco and 4R FPRs is just a clean air vent for the atmospheric side of the FPR diaphragm. Fuel pressure is fixed at around 43 PSI. It might also provide a degree of safety should the diaphragm leak.