1988 R20 Suburban won't stay running

[dwntwn]

I haven't had this vehicle too long—it needed a few things like tires and battery but its a pretty clean, well maintained vehicle. In the first week I replaced the coil, plugs, wires, cap and rotor and its ran great.

Last Friday after a 75 mile trip out of town it wouldn't start without a lot cranking, its never done that. When it started it wouldn't stay running for more than 10-15 seconds. The injectors are pumping fuel while it runs, then it cuts off.

I replaced the fuel filter and fuel pump (Delphi). No change.

Confirmed the grounds at the fuel pump, water neck, pulled the starter to inspect all the wires where they are near heat, no breaks.

I replaced the oil switch and sender thinking possibly the switch wasn't sending the pressure reading to the ECM, no change.

I have tried the paperclip trick but was unable to retrieve codes. Not sure why that is. I replaced the ECM thinking that was the problem, but it still won't stay running and is hard to crank.

I just replaced the IAC, which seemed to make a difference at first—the idle was higher but the engine still cuts off after running a few seconds.

Not sure what to think—this truck ran so good and now it won't, its got me convinced it HAS to be something simple (its got 3/4 of a tank of new gas) but I'm stumped.