1993 F-150 with dual tanks. It seems to only pump fuel from the rear tank
a couple weeks ago I pulled the selector switch out and verified that it was working
with the pins jumpered at the EEC connector I was able to hear both pumps turn on. With it in the front tank
position I went and drove 50 miles. The fuel gauge never dropped. Switching tanks and the rear tank had went down. I have verified that the front tank is still full when stopping at the gas station
Today I decided it was time to tear into it. Pulled the box off to get a to both tanks
the front pump is definitely running. Pulled the fuel lines off. The pressure line was squirting fuel out. With pressure line connected and return line disconnected fuel was returning to the front tank. Unplugged the rear pump to make sure both pumps weren’t running at the same time and to verify that the engine would run from the front pump
disconnected the front pressure line again and activated the rear pump. Fuel was squirting out from the front line
from what I read this should be expected as the check valve is in the pump

connected the front pressure line and disconnected the return line

with the rear pump running fuel was coming out of the front return line. Which again I believe should be normal



but I have yet to experience either tank running over from fuel returning to the wrong tank.
if the front pump is running the gauge should go down

It’s been slow to start since I got it and fuel rail pressure drops to zero immediately on shut down, so am already thinking bad check valves
but the not sucking from the front tank has me baffled

 

Unless the fuel level sender is wacked, I had one that would sit on full until the tank was half empty and then it would drop like a rock.

Does the engine oil smell of gas?
Any fuel in the vacuum line attached to the FPR?
Any raw fuel in the charcoal canister or the vacuum lines attached to the throttlebody?
If you have none of those problems then there is something up with one or both FDMs, check valves in these should prevrent crossflow and retain some pressure in the system over the short term, the pressure might bleed of eventually but it should take many hours or even days.

 

No fuel in the Vacuum lines. I just changed both pumps, but haven’t drove it to test it yet

 

It's very possible the fuel sending unit wires are swapped at the switch. These trucks are 30 years old and every owner has messed with them.