The returnless fuel system really wasn’t. They just mounted the regulator back at the pump. But since it wasn’t vacuum referenced I’m not sure if the ecu was different. View attachment 1716191152
The only real difference is the “returnless” style expects a constant fuel pressure while the return style with a FPR on the rail had a variable pressure depending on the amount of vacuum. And the PCM‘s were tuned to match. So if you run a PCM that used a “returnless” style, just leave the vacuum disconnected from the FPR since the PCM is tuned to expect a constant pressure.
Plenty of people have mixed and matched while ignoring this and haven’t had huge issues. But I think they run better if you take it into account. Kind of like running larger injectors and not tuning for them, it can work but maybe not as well as it should.
