That is true, and it makes me wonder how the later systems are more advanced at controlling fuel. I've read enough of how the newer PCM's are faster and easier to tune radical/boosted combinations. I haven't dug deep enough to find out if that's preference by tuners or idiots, or actually advantages of the later models. Can a later PCM operate a return style fuel system or does it have to be return-less. Interesting ....
@CDW6212R
I see no reason why a later model PCM could not run with a return type system, so long as the proper pressure were maintained at the rail. Early return-type used a simple vacuum-operated pressure regulator with a diaphragm which operated a valve opening into the return line. The pressure side of the valve was exposed directly to fuel pressure in the rail. As vacuum rose higher, meaning the engine was working under less load, less power called for, less gasoline needed, the diaphragm opened the valve farther, dumped more fuel into the return, thus holding pressure in the rail fairly constant under all engine demand conditions. How constant? Not very, as anyone who has watched a pressure gauge hung on the Schrader, might go up and down from 30 to 45 psi. That caused big variation in air/fuel ratio, harder to control, the difference being "covered" in closed-loop by the O2 sensors. EEC-IV (OBD-I) did this, running at around a 40 psi baseline, worked fine, as far as I was concerned. I used that system to power 8 or 10 conversions, a Maverick, 1/2 ton Toyota, 68 Mustang 351W, '70 Bronco, '79 Ranchero, even took a '71 F-150 with 390 and bored and mounted injectors on a home-made fuel rail set on injectors mounted in the ports of the cast iron manifold. It's crazy what you can do, if yer crazy!
This late '90s returnless scheme I knew nothing about, not even it's existence, until several really sharp guys here straightened me out......Then I bought a 2004 Ex, 4.0 SOHC, and learned Ford had added the pump speed control idea, maybe early in Gen III. I know they had a "fuel pump Module" before '04, which took the FPTS data and converted it into a varying voltage controlling pump speed by a feedback system: if rail pressure is constant, pump runs at a constant speed. Rail pressure drops a teeny bit (how much, I don't know, but probably several psi), or rises, as engine demand changes, the pump speed is changed to bring the pressure back to desired level. Slowed down for a short time, speeded up for a short time. How often? Dunno. But do know varying speed of DC motors every few seconds shortens motor life.
So why the hell go to the trouble? Asking for more places for things to f$ck up. True. BUT, mine has 165K on the original pump Ford put in the tank! Can't hear it run, yet. Longer pump life because the pump doesn't push maximum output all the time, as returnless did.
What else? PCM now has more control over programmability: added road-expected strategies can be used for varying F/A ratios for a variety of conditions not possible before. That don't impress me much.........imp
