Spent all summer reading posts about recalls and cross feeding. Recall was done before I brought the truck. So I replaced both external check valves but front pump still pumps to rear tank. What keeps the return fuel flow from going to both tanks (that's assuming the external check valves are on the two fuel supply lines)? The truck runs fine on either tank/pump. So I fill both tanks and use the rear one till nearly empty and then select the front tank. I am able to go about fifty miles before all of the fuel in the front tank is pumped to the rear tank, so I re-select the rear tank and use it until it is empty, then refill both tanks.
I sketched the fuel diagram (as I understand it) with external check valves in the pump discharge lines, both going to the fuel rail and both return lines, each going to its respective tank. There has to be something to isolate the live return fuel line/tank from the non selected one. ???

 

The shuttle valve in each tank only lets fuel in while its own fuel pump is running.
You more than likely do not need the two out side check valves as they were a band-aid fix for a bad run of fuel pressure regulators that would blow out the non running FDM check valve.

Fuel Pump Circuit Operation:
When the ignition is switched to the ON position, it turns the EEC Power Relay on. The EEC Power Relay provides power to the EEC-IV processor and the control side of the fuel pump relay. Power for the fuel pump is supplied through a fuse link or high current fuse attached to the starter solenoid (battery side). From the fuse link or high current fuse, current flow is through the fuel pump relay and Inertia Fuel Shutoff (IFS) switch to the fuel pump. The IFS switch is a safety device used to shut off the fuel pump in the event of a collision. If the IFS switch is "tripped," it must be reset by depressing the white or red button on the top of the switch. The fuel pump relay is controlled by the Powertrain Control Module (PCM).

When the ignition switch is turned to the ON position, the fuel pump will operate. If the ignition switch is not turned to the START position, the PCM will shut the fuel pump off after approximately one second. The PCM will operate the fuel pump when the ignition is in the START position to provide fuel while cranking.

After the engine starts, the PCM will continue to operate the fuel pump unless the engine stops, engine speed drops below 120 rpm, or the IFS switch is "tripped."

The Fuel Delivery Assembly (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.

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 either moulded or welded into the tank or 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 filter screen on the fuel rail port of the injector is part of the fuel injector assembly and cannot be serviced separately.
3. The filter screen on the fuel inlet side of the pressure regulator is part of the fuel pressure regulator assembly and cannot be serviced separately.
4. The fuel filter assembly is located between the fuel pump and the Schrader valve or injectors. 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.



Fuel Pressure Regulator:
The Fuel Pressure Regulator (image below) is attached to the fuel supply manifold assembly downstream of the fuel injectors. It regulates the fuel pressure supplied to the injectors. The regulator is a diaphragm-operated relief valve. One side of the diaphragm senses fuel pressure and the other side is connected to the intake manifold vacuum. Nominal fuel pressure is established by a spring preload applied to the diaphragm. Balancing one side of the diaphragm with manifold vacuum maintains a constant fuel pressure drop across the injectors. Fuel in excess of that used by the engine is bypassed through the regulator and returns to the fuel tank.




Fuel Sender:
The fuel sender consists of a variable screened resistor made up of a ceramic substrate. It is controlled by the action of an attached float in the fuel tank. When the fuel level is low, resistance in the sender is low. When the fuel level is high, the resistance in the sender is high. As the float moves from empty to full, the resistance will gradually and continuously increase.



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