1992 F150 Dual Tank problem

My truck runs a while and then the check engine light comes on. It stumbles and can’t even move from a dead stop. After it cools down it runs fine. Everything checked out except the fuel line pressue, it dropes 20 psi when the light comes on, so I replaced the fuel pump on that tank. It still does the same thing. I have checked the inertia switch and it is ok. The relay was chattering so I replaced that also but the problem won’t go away. Any thoughts?

Get the code(s) from the diagnistic connector. This is pre OBDII so you can get them by shorting out two pins in the connnector and counting the flashes of the CEL. YOu can probably find out the codes and what pins to short on line somewhere or check a repair manual in the library.

The fault codes were odd, they indicated the computer was bad. So I have checked out all of the systems. When the check engine light comes on the fuel pressure drops off 20 psi and the exhaust is sooty. I replaced the fuel pump and it still did it. Then I measured the input voltage at the fuel pump relay and it dropped to 3.7 volts when the check engine light came on. I then jumpered to a switched on signal and now I get the check engine light rarely but I can’t find a 4 way valve. I think there is a check valve assembly that may be bleeding off pressure. Do you know where that may be located?