It doesn't, and just runs a set pressure, accounting for additional demand via injector pulse length (opening for longer per cycle). I believe the regulator is in the pump module on returnless systems.
Some Fiero guys run the Corvette style fuel filter, to limit the lines to the engine, and it has the return line from the filter to tank. You can also swap to a Series 2 return-style rail, and add a boost reference setup. Up to you.
I just checked my fuel pressure and it seems really messed up. KOEO it goes up to about 45psi and then drops down to 25. Within the next few minutes it slowly drops to 10. I checked it while running and it's only at 44psi. It's nowhere near the stock 52psi. Where it has no return line I would think the injectors were leaking down but there is no hard starting problems. There are no leaks on the ground or smell of fuel so I'm lost on what's happening.
Have you traced the fuel lines back to the tank, and figured out what the overall system components are, yet? Before trying to figure out what an issue might he, you'll want to know each component involved, especially in a custom setup.
I finally was able to check the whole system out. It has an adjustable fuel regulator (with vacuum connected) mounted underneath the car and they put a return line to the tank. Now as far as adjusting, it seems to be a crap shoot. All other FPR's I've had KOEO stay steady so you can adjust it, but this one starts bleeding off all the pressure 2 seconds after KOEO.
Our computers use a 2sec prime when you key on, then it'll run normal when the engine's on. If it's reducing pressure when running, that's an adjustment you may need to make based on a gauge reading at idle.
40-50 ballpark, never dealt with adjutables. I run the stock 3.5 bar (~50.7 psi) or Audi/VW 4.0 bar (~58 psi) version from early 2000's. I don't remember if vacuum at idle/cruise pulls it down some on stock Bosch style, or just boost raises it above that setting.
This just gets stranger. I cranked the FP up from 44psi at idle to 55psi. The idle seemed to smooth out a little and now at full boost I'm getting 10 vs. only 6 before. I wish that would happen with my turbo cars....lol.
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