[BillGrissom]

A little more info on fuel pressure regulation. Earlier MPFI systems, up to ~2000, generally had a fuel pressure regulator on the injector fuel rail. Those had a return tube back to the fuel tank. The regulator was a simple "back-pressure" design, similar to a spring-loaded poppet that opens progressively as the upstream pressure increases. However, a vacuum hose from the intake was plumbed to it so it controlled the dP between the fuel rail and intake (thus, internally a 2-sided diaphragm that opened a fuel valve). The idea was to control the pressure drop that mattered most - across the injector. An EPA mandate required a change to "return-less" fuel systems, with a single line to the engine, to avoid heating the fuel in the tank and increasing evaporation. Thus, they could only control the pressure right at the outlet of the fuel pump, within the pump assembly (in the tank). In practice, it is simpler, safer, and works almost as well since the computer's equations can account for drops to the fuel rail, the computer already knows the intake pressure (PMAN sensor), and fuel is ultimately controlled by O2 sensor feedback anyway. I wonder if the pulsation damper simply physically replaced the pressure regulator of earlier cars, so they didn't have to redesign the fuel rail and because the regulator diaphragm had acted to damp pulses. Besides the fun of speculating about design choices, the main take-away for me is that it is more important to change your fuel filter in a return-less design since the engine computer assumes a certain pressure drop to the rail, instead of it being controlled as in the past.