Ford Electronic Returnless Fuel System

By Glen Beanard technical contributor

Why?

Here we go again, a simple electric pump pushing fuel up to the engine, with its

pressure being controlled by a little valve, and returning the unused fuel to the tank was

not good enough. Now, we need to make that all fancy by adding a new controller,

adding a new sensor and speeding up and slowing down the fuel pump to control

pressure. What sense does that make? Are they just trying to sell new parts and tools or

what?

Well, actually, the main idea is to reduce fuel vaporization back in the fuel tank.

With the older return type fuel system, the fuel would be pumped up into a hot fuel rail

overtop of a hot engine, and the fuel would return to the tank full of heat energy.

Naturally, the heated fuel would vaporize in the tank at a higher rate than if it were kept

cool. That s the idea; no return line to send extra heat energy to the tank.

As an additional benefit a returnless system, having one less fuel line reduces the

chances of a fuel leak. Also, making it electronically pressure controlled, gives the PCM

more flexibility over fuel control by adding another option if needed. With an electronic

returnless system, the PCM can now raise or lower fuel pressure at will.

Theory and Operation

This system consists of a rotary vane fuel pump (same as before), fuel line

(pressure only), fuel rail pressure sensor (that also contains a temp. sensor), fuel pump

control module, and of course the PCM. This much should remain the same for any

electronic returnless system even when compared to other make s versions. A couple of

things that become optional across other makes, that Ford still includes, are the fuel pump

relay and the in-line fuel filter. One Ford specific component that still remains after more

than 2 decades is the inertia fuel cut-off switch. One notable change however, is the fact

that the PCM no longer controls the fuel pump relay. The fuel pump relay is now toggled

by the ignition switch.

It s a system with a simple goal; maintain pressure in the fuel rail. It just has a

high tech twist on achieving that goal, when compared to its older brother. The PCM uses

the fuel pump control module to electronically maintain the desired fuel pressure in the

fuel rail. Fuel pump on command is still, overall, the same as before; 1 to 2 second

burst-on during the initial key on event, and again after a crank position sensor signal

is received by the PCM, as during start-up, cranking, and engine running. While running,

the PCM is attempting to maintain 40psi of pressure at the injector nozzles. The PCM

will raise its target above that pressure if it sees the fuel temperature rise high enough

to cause vapor pockets in the rail. The higher pressure counteracts vaporization.

To achieve its goal, the PCM communicates a duty cycle to the fuel pump driver

module (FPDM). Between 5% and 50% duty cycle is the normal range of demands from

the PCM to the FPDM. In this range, the fuel pump output is directly proportional to the

fuel pump s on time. Whatever the PCM s duty cycle is, multiply that by 2 and that

is the fuel pump s on percentage. For example, a duty cycle from the PCM of 20% will

equal 40% of fuel pump full-on, and a duty cycle of 50% would equal 100% of full on at

the pump. (see drawing #1)

If the PCM wants the fuel pump turned off, it will send a 75% duty cycle to the

FPDM. However, the FPDM will accept 67.5% to 82% as a valid off command. As for

the duty cycles that fall between the cracks, 0% to 4%, 51% to 67%, and from 82.5% to

100%, they are invalid. The PCM will not send those duty cycles unless something has

gone terribly wrong inside the PCM, or unless a technician commands the PCM to send

those duty cycles via suitable scan tool. Any of those signals from the PCM will result in

zero fuel pump operation. (See drawing #2)

The FPDM controls the fuel pump according to the PCM s command, and

provides and diagnostic feedback to the PCM. On the Thunderbird and the Lincoln LS6

and LS8 however, there is no actual FPDM. Instead, role of FPDM is performed by the

Rear Electronic Module (REM). The REM receives commands from the PCM and

controls the fuel pump in the same manner as the FPDM would, except that it receives

commands through the data bus. The Ford GT40 uses 2 FPDMs do to its duel injection

fuel delivery system. The duel FPDMs share a single output from the PCM, yet have 2

separate diagnostic feedback circuits to the PCM so that it can monitor them separately.

In accordance to the command from the PCM, the FPDM modulates the fuel

pump ground to rapidly turn on and off the fuel pump. The rapid on and off

switching, speeds the pump up or slows it down to maintain fuel pressure demands. The

FPDM communicates system conditions back to the PCM in the form of a duty cycle

signal. The FPDM monitors its commands from the PCM and the fuel pump secondary

circuit. It continuously sends a signal back to the PCM that defines the state of health in

the circuits between the PCM and FPDM, as well as the circuit for the fuel pump.

The fuel rail pressure sensor (FRPS) is a diaphragm type strain gauge devise that

measures the difference between the fuel rail pressure and the internal intake manifold

pressure. It is referenced to MAP pressures, instead of atmospheric pressures, so that its

value reflects the fuel pressure at the injector nozzles. The PCM measures the voltage

drop across the terminals of the FRP sensor. The internal resistance of the FRP sensor

increases as the pressure increases. As the resistance climbs, so does the voltage drop,

and then so does the fuel pressure PID. The FRP sensor also houses an engine fuel

temperature sensor (EFT). The PCM is also looking at voltage drop at the EFT as well.

Keep this in mind, since the majority of circuit wiring problems (such as broken wires,

unplugged connectors, loose or corroded pins, etc.) result in high resistance. If your fuel

pressure PID and/or Fuel temp PID are too high, don t forget to include sources of high

resistance in your diagnostic checks.