Hey Scotty! Thank you for answering all these questions! I have a 1989 Grand Marquis that is having a general performance problem. Sometimes it dies after starting when started warm. It will always run rough for a few seconds when started warm. It also is a little sluggish accelerating from 45 to 55 mph unless I shift out of overdrive into 3rd gear some thing between 35-45 mph unless i go into 2nd. I replaced the fuel filter that was very constricted and it stopped dying as much but still does occasionally. The non-runing fuel pressure is 36 PSI (35-45 is normal) but the running fuel pressure is 28PSI (30-40 is normal) I removed vacuum from the fuel pressure regulator and the running pressure was 38PSI. With the regulator disconnected, it seemed like the acceleration was a little better but only marginally. When I rev the engine the pressure initially increases to 30 but then drops back to 28 after the engine has stabilized at the higher RPM. It sounds like the fuel pump is weak (sometimes I hear the thing squeaking when its priming the system) but I want to know if the test I did can rule out if the fuel pressure regulator is to blame. or other things to check out.
I would bank on the fuel pump being weak, dead head pressure it or just replace it