Sorry if this is a redundant post, I've been driving my 5.3 nv3500 swapped 90 Sierra for about 3 months now. I finally ironed out the OBD2 sensor and realized I had two codes, P0174 and P0171. I bought a fuel pressure gauge and read 25 psi at the fuel rail, key on engine off. I tested for vacuum leaks, and couldn't find any. I had a small leak at the C5 regulator which I fixed, the guy who installed the AN fittings (me) didn't do a good job tightening them, go figure. I tested again, and got 25 psi still. The pump (ep381) is brand new and was replaced when I swapped the engine. The regulator is brand new too, so I'm lead to believe it's one of those, but before I bought a regulator, I wanted to hear from the experts.