The way I understand it, In the return-less fuel system the fuel pump makes a constant pressure of around 65-70 PSI. The FPR in the tank (which is located right above the fuel pump) returns anything above that pressure right back into the fuel tank. Hence the fuel pressure at the rail remains close to a constant pressure and the PCM regulates fuel injector pulse rate and width based on feedback from various sensors (like the O2's, MAF, air temp, coolant temp). The damper on the fuel rail buffers the fuel pressure as needed based on engine vacuum.
The FPR in the tank is strictly a mechanical device, which uses a spring to route excess pressure back to the fuel tank. The in-tank FPR is a simple, reliable device which rarely fails. If it does fail it's typically because its plastic housing cracks. Other than the fuel pump itself the most common fuel pressure problems are due to splits in the in-tank submersible fuel lines.
@koda2000 @shucker1
We are having a discussion here in which terminology and names for components are clouding the actual physics of it. This has left
@1998Exp as confused as me!
So, to further cloud the issue: The fuel pump is a constant-volume pump, meaning each revolution of it's guts produce the SAME amount of flow. Fuel is incompressible, so the pump is designed to deliver enough fuel when the engine is "balls-out", and at that time if it were sized with NO cushion for more, the REGULATOR in the tank would be bypassing NO FUEL back to the tank. This regulator is set to allow only a maximum pressure to exist, to protect the pump from overload. A fixed-volume pump having it's output physically blocked off, will either BREAK, burst it's connecting hose, or stall it's driving device, in our pumps, an electric motor. This is due to the incompressibility of liquid.
The above explains in part why earlier EFI systems used lower fuel pressure, perhaps 40 psi. The newer "idea" uses about 60 psi, if i remember right, but the thing here which applies, is that to run "returnless" successfully, the pressure relief in the tank must establish the actual fuel pressure used by the injectors. Say the throttle is suddenly tromped to the floor.....INERTIA of the fuel in the line to the rail, maybe a cupful, prevents an instantaneous increased flow of fuel to the rail. It needs that added fuel BAD, and NOW, or a BOG is felt; thus a "damper" on the rail, which I stubbornly still call a "pressure regulator", since if pressure drops suddenly, that means DEMAND for fuel has jumped up, manifold vacuum drops, and the damper opens to allow more fuel flow into the rail. It's not much different than the return type system, except it works BACKWARDS: instead of closing off the return port to get more fuel, it opens the INLET line (the only line there), to admit more fuel volume until demand drops, inertia loses it's grip, more movement out of the tank happens, and this type of repeating process goes on and on.
Apparently, something moved Ford to re-work the returnless, but not go away from it, by quitting trying to control rail fuel flow volume in this way, and rather began controlling the volume (and therefore the pressure) by controlling pump speed, I guess in 2004, maybe. Mine has it, '04. Relief valve in tank, never opens under normal circumstances. It's setting is higher, I've heard about 100 psi, again this to protect the pump, but maybe more importantly to "cover-ass" for failure of either the rail Fuel Pressure and Temperature Sensor (which "tells" the Pump Driver Module how fast to run the pump) or failure of the Module itself. Either would leave the vehicle dead in the roadway.
So, PCM senses such failure, electrically of course, bypasses the sensor and module both, and runs the pump full-bore at 100 psi, the engine might smoke like hell, and sure pollute, but not kill the vehicle, which might be more dangerous than the Environmental Lawyers.
Now if you guys believe this load of bullshit, I'll qualify to charge double tomorrow! imp
