While troubleshooting my rough running 89 F250 5.8L, I finally checked my fuel pressure at the rail.
+/- 25psi, +/-32 with vacuum disconnected fro regulator.

I actually lost fuel pressure a time or two and was able to determine that the frame mounted pump was running, but the in-tank pump was cutting out every now and then, all while the pump test plug was grounded.

Can a faulty in-tank pump cause low pressure at the rail?

 

Most of time not unless you have a dirty filter in the selector valve.

 

Good point, SubFord. I have a fairly fresh frame mounted fuel filter and selector valve filter. Also, my side tank has been removed an the fuel lines capped. I'm running off the rear tank only, through the selector valve.

 

I can confirm that a weak in-tank pump can cause low pressure at the rail, even with a good working frame pump.