My Haynes manual for my 2003 F150 4.2 lists two systems in the fuel system section. ( I need to test fuel pressure) The date of use for the return system is 97-04 and returnless 2001 and later. Overlapping dates for Return vs Returnless and no instructions to show the difference. Does the 2003 4.2 use a return or returnless system? The fuel pressure spec differs on each system. Thanks

 

You would have 1 fuel line and no regulator (its inside tank), instead of a regulator on fuel rail and return line to the tank. Regulator will have vac tubing on it. As engine speed increases and manifold pressure rises, fuel pressure is increased to compensate for lack of vacuum in the intake. Keeps pressure drop across injectors more constant with engine speed.

After 2004 it was an electronically regulated system.

I cant imagine the spec values are much different for
The two in 2003.

Looks like slightly higher min at idle, 35 vs 28, for returnless.