I just spent some time with the electrical portion of the '89 FSM. I haven't had a similar problem but it's a good time to attempt to understand the fuel circuits. There's a lot that happens within the ECU that I can't see without an circuit board diagram but the associated wiring schematic shows two (2) circuits within the control relay:

Current flows from one set of contacts on the MPI control relay to the MAF sensor, the ISC, the purge control solenoid valve, the EGR control solenoid valve, the injectors and the ISC. The trigger for that circuit appears to be the "light green" wire from the ECU to the control relay.

The other set of contacts powers the fuel pump drive terminal with supply voltage from the ignition switch. The trigger appears to be the "white/red" wire from the ECU to the control relay.

You can check voltage on those wires before and after the engine quits to see if there's a change. However I'm very suspicious of the ground connection on the control relay. Check continuity on the "black" wire coming from the control relay to ground. If you read anything other than zero, you may have found the problem. If you're using an analog meter, be sure to first "zero" the meter.

If the control relay harness/connection passes the ground continuity test and there's voltage on the 2 identified relay trigger wires - if it were my truck it would then be time to get out the dynamite. <img src="/forums/images/graemlins/lol.gif" alt="" />

I know there are differences in the '89 ECU and your own, but I don't know what they might be. The setup could be quite different and the wire color markings certainly could be so it would be better to use the manual for your model year. The ground wire on the control relay would be the same "black" color, though. Maybe someone with a '91 manual can verify the wiring as identical or not.... in any event, it won't hurt to measure voltage on those wires.

Good luck,
Frank