1988 F150 Lariat SB 5.0 EFI Dual Tanks.I have the "Low pressure pumps" in both the Rear, and Side tanks, and a Hi-Pressure Pump" on my frame rail- Now, I Understand that I have a "Fuel Resevoir " of sorts that holds un-used fuel that was sent to the Fuel rail,Does anyone know much about this system, ?? I believe I have some contamination in the rear Tank, It has about eight gallons of gas in there, -when the motor begins to sputter, I switch it to the Front tank, and it straightens out, stops sputtering, and poppin'Switch it back to the rear tank, and within a couple of minutes, it starts sputtering agin. Then I click it back to the front to keep it from dyin.I dont use the front tank much because it has a leak, But for some reason, after a couple of days or so of driving, It seems to accumulate Fuel..?Is this the Fuel being sent back to the Fuel resevoir, and back into a Tank?Because It seems that whatever is in the rear tank, is slowly making its way into the front tank, possibly from this By-pass to the Fuel Resevoir?Can anyone clarify this for me? Thanks

 

That "fuel reservoir" Is also called the 6 port reservoir switching valve. It is before the High pressure pump, and has 2 lines to each tank and 2 to the engine, 1 supply and 1 return each. When that Valve fails, you get 'cross flow' which is mostly return to the wrong tank, but can be pumped in from the other tank. Check the wiring to the rear sender (low pressure), and if you think the rear tank is contaminated, get that gas out. The cross flow means you need a new selector valve, but if you have no intention of using the midship tank, you can bypass it.
-Mike

 

I see how this works, -thanks mike, -gotta get the bad gas out.