You should check out http://www.thirdgen.org
. You will find lots of helpful posts there. There is an diagnostic and adjustment procedure for those, though I'm not familiar with the 89 2.8 specifically. So take this with a grain of salt since your setup may be different. From what I remember, the diagnostic starts like this.
Remove the IAC connector, unscrew the IAC and reconect the wires. Then have your assistant put the ECM into diag mode by shorting the upper right two connectors on the diagnostic port. Turn the ignition on but don't start it. The IAC motor should fully extend and then back up 144 counts (3/16" or so). Once it has done this, turn off the ignition, remove the diag jumper, disconnect the wires, reinstall the IAC motor, but don't reconnect the wires just yet.
Some of those engines required an adjustment on the throttle positon sensor which would affect idle if it was off. With the IAC motor in the preset position from above, start the engine and adjust the idle stop until you have about 700 RPMs. The engine should be warm / hot for this. So let it run a while if the engine is cold. Once the RPMs are set, turn the motor off. Then turn the ignition on, and measure the TPS voltage (center pin) to ground (right pin? but not the chassis). The TPS sensor can be adjusted slightly by loosening the two attaching screws. Adjust it for about 0.7 volts. When you are done, turn off the ignition, reconnect the IAC motor (make sure the diag jumper is removed) and start the motor.
If this doesn't help or work out correclty, you'll have to dig in deeper. Most Hanes or even Chilton service manuals should have some troubleshooting procedures for the IAC circuit. Better yet is to get your hands on a professional service manual like Mitchel's. Your local library may have something.