I have a 1990 F-150 XLT Lariat 2WD with dual tanks and I have some questions about a switching issue. I can run on the front tank just fine it when I switch to the back tank, driving or just idling in Park, it wants to die immediately. I first though that it could be the selector valve but I looked under my truck and there isn't one. All of the wiring and/or gas lines look to be as they came from the factory. How else would Ford have done the switching if there is no selector valve? Is it just the selector switch on the dash causing the issue? My back tank has fuel in it and when I switch tanks, the gauge shows full as it should. It's almost like fuel isn't being allowed through for some reason. I'm assuming a clogged fuel line(s) from the back tank? If that is the case, I know I'll be having to drop the rear tank. Also, the fuel gauge on the dash shows 'Empty' when I first start driving but eventually works its way up to showing proper fuel level after driving for awhile. Is that a sign of a bad fuel gauge? Thank you in advance to anyone who can help.

 

They changed the system around the time of your truck. Earlier trucks had a third pump on the frame rail, high pressure. Low pressure pumps in the tanks. Later trucks had a high pressure pump in each tank. On the later system the "in tank unit" has a valve also for the fuel return. I don't remember if it is part of the pump or what.

Some trucks it's easier to lift the bed. Some have stuck bolts. I've had good luck lowering tanks with tie down straps. Full tank is a wrestling match. Might outweigh you. Spilled gas can hurt you. Don't go in the tank if you don't have to.

It could be the switch, wiring, pump, etc. Sounds like the sending unit is bad, or wiring. The tank might be trashed with rust inside.

 

Thank you so much for your help. At least I'll have some direction as to what to do. So how are the tanks switched if there is no switching valve? When I turn the key no matter which tank it's on, I hear the sound of the fuel pump(s) so I assume they're still good. I'm guessing there's only one fuel pump but I'm not sure. Thank you again.

 

The valves in the tanks prevent crossfeeding between tanks. They also route the fuel return to same tank. The switch powers up the appropriate pump and the valves route the fuel to prevent crossfeeding.

 

The Fuel Delivery Assembly AKA Fuel Delivery Module (FDM) consists of the fuel pump inside a reservoir that is attached to the pump and sender assembly. In a two tank system there is one assembly per tank. The sender assembly handles the switching of the fuel through internal valves. To service the assembly, replace it.
The in-tank mounted fuel delivery module containing the fuel pump is capable of supplying 105 liters (27.7 gallons) of fuel per hour at a working pressure of 270.0 kPa (39 psi) for the 5.0L, 5.8L and 7.5L MFI engines. The fuel delivery module can supply 90 liters (23.7 gallons) per hour at 380 kPa (55 psi) for the 4.9L MFI engine. The fuel pump has an internal pressure relief valve to provide over-pressure protection in the event the fuel flow becomes restricted. Over-pressure is restricted to 850 kPa (123 psi) and reduced fuel flow will result. The system pressure is controlled by a fuel pressure regulator (9C968) on the engine.

Attachment 280308

Attachment 281295

Reservoirs and Filters
Fuel reservoirs are used to prevent fuel flow interruptions during extreme vehicle maneuvers with low tank fill levels. The reservoir is located in-tank and is part of the Fuel Delivery Assembly.

Filters and Screens
The system contains four filtering or screening devices:
1. The fuel intake sock is a screen located on the intake side of the fuel pump and is part of the fuel pump assembly.
2. The fuel filter assembly is located between the fuel pump and the Schrader test port on the injectors fuel rail. This filter should be serviced periodically by replacing it.

Shuttle Selector Valve
The Shuttle Selector Valve opens the return line to the tank when the fuel pressure on the shuttle diaphragm exceeds the opposing spring force. The return flow is directed to the appropriate tank on vehicles equipped with dual tanks.

Supply Check Valve
The Supply Check Valve, which is normally closed, opens when the outlet pressure from the energized pump exceeds the opposing check valve spring force. When the pump is de-energized (i.e., engine is shut off), the supply check valve closes to maintain pump prime and fuel supply line pressure.

Venturi Jet Pump
A portion of the high pressure flow from the pump is diverted to operate the Venturi Jet Pump. The Venturi Pump draws fuel from the tank into the assembly. This process ensures an adequate fuel supply to the pump during extreme vehicle maneuvers and steep vehicle attitudes with low-tank fuel fill levels.
The inlet of the Venturi Pump has a nylon filter on it to prevent dirt and other particulate matter from entering the system. Water in the fuel tank can pass through the filter in small quantities.

Test Point
A pressure test point with a Schrader fitting is in the engine fuel rail to relieve pressure in the fuel system and to measure the injector supply pressure for service and diagnostic work.
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Bad assumption, a failing fuel pump is the most likely cause of your problem. Get yourself a fuel pressure gauge and test pressure for both tanks at the fuel rail, my guess is you will have very low or no pressure from the rear tank.