I've previously replaced the stock TBI fuel pump with the later model EP381 and with the mildly modified LO5 engine I think I managed to get 15.0 PSI from it at the time, which was all I needed.

When I changed to a Vortec crate motor with TBI on it I also went with an aftermarket 46mm throttle body with an adjustable fuel pressure regulator, I never had more than 15.5 PSI showing on the fuel pressure gauge even though the regulator was apparently set to 18 PSI when it was sent to me.

I had a go at cranking up the fuel pressure by winding up the adjusting screw but it didn't seem to make any difference at the time.

I normally get between 15-15.5 PSI constantly but my W.O.T DC injector % indicates I need more fuel pressure (Wideband AFR's are okay at W.O.T.).

Tonight I thought I'd have another go at increasing my fuel pressure, initially I wound the adjusting screw up another couple of full turns and changed the spring to one out of a stock throttle body that netted a drop to 13 PSI. I then without making any other changes put the spring back in that came with the adjustable regulator and I got 15.5 PSI again. I don't seem to have a lot of adjustment left and I've probably wound the screw up 5 full rotations without any noticeable change in fuel pressure. Anyone have any idea what might be going on, I'm assuming an EP381 can crank out more than 15.5 PSI?