I set my fuel pressure to 40psi, then check it a few days later and it's at about 45psi, so I set it to 40psi again, check it again a few days later, and its back at 45psi. Why is this? I tighten down the nut after setting it, so how could the pressure be increasing by itself?
Dude, mine does the same damn thing. I posted on here when I put mine in and nobody had any really good input. The best I can figure is that because the computer is always changing the pulsewidth of the injectors depending on conditions (engine temp, ambient temp, barometric pressure, etc...) its always gonna vary. The best you can do is set it at full operating temp in the middle of the day and leave it, it should be the same for that specific set of operating conditions after that. I set mine at 42 psi at full operating temp sitting in my driveway at idle. But when I start it in the morning it always reads about 50psi until it gets fully warmed up, and when I run it hard it reads anywhere from 38-45 psi... I guess it works ok cuz my check engine light never comes on so the computer must be doing ok with the pressure reading all over the map. The best thing to do is get a good A/F ratio guage and use that to set the pressure depending on what your engine setup is.
How are you setting the fuel pressure? If you're not doing it this way already, then try the following:
After letting the car fully warm up, pull the vacuum line from the adjustable fuel pressure regulator and adjust it to the desired setting. Now reconnect the vaccum line. Check it out afterwards and see if it changes.