After owning an '89 with the 2.8L, with the exact same problem, and having the same frustrations over replacing every part imaginable, I came across something in my research of the wiring diagrams. The Coolant level sensor was tied into the same 5volt wire as the IAC & MAP, causing those to read as malfunctioning. You can test this theory by simply disconnecting the pigtail connector to the Coolant Level Sensor on the right (passenger) side of the radiator. If it resolves your problem, then replace that sensor, (with a new one that is NOT Borg Warner), and you are back up and running. Hope this helps!!
