Well, now I'm not sure what's going on. When I got home from work yesterday I pressurized the system and pinched off return line after the FPR. A couple of hours later, it was still at ~35psi. Probably not the pump's check valve, then. Good - it's a brand new, out of the box pump! So I unclamped that hose, repressurized, then clamped the send line. A couple of hours it was holding steady at ~35 psi.
Weird. Still not ready to give it spark though, being worried about a potential fuel leak. By then it was late so I unpinched the hose and went to bed. This morning, with no clamped fuel hoses, pressure was still at ~35 psi.
I'd chalk this up to a faulty gauge, but the pressure loss was recorded by two different gauges. There was a seeping leak at the gauge during the first run of tests but I've since resolved that, with pressure loss still recorded afterward. Other than tightening that connection, clamping the pressurized hoses, and putting a vacuum hand pump on the FPR, nothing has been done to the components. I haven't fired the engine, so I doubt if an injector was stuck that it would become unstuck, if that's even possible. Haven't swapped any other components, not even a hose clamp.
Any theories as to what's going on? MAYBE the FPR diaphragm or spring is bad but not broken, and making it work by increasing the fuel pressure (overpressurizing by deadheading the return line) or applying more vacuum, via the hand pump, than it would normally experience kinds sorta fixed it? That might explain the intermittent periods the car would actually run decently well. And is it possible that a bad FPR diaphragm could cause a significant vacuum leak, enough to drop my gauge to the levels it was displaying? I doubt it, but I really don't know.
as good as a car can be... briefly.
Last edited by chairmankaga; 04-12-2017 at 08:32 AM.