First, pull out your intake boot, and then unplug the throttle position switch. You'll see three prongs, with a multi-meter test for continuity between the left one and the middle. If there isn't you'll have to remove the throttle body to adjust or remove it. More on that after you get back.
If there is continuity, then remove the top panel above the glove box, just behind the computer to the left you'll see a green box, about the size of a bar of soap. This is your idle control module. check the plug for dirty contacts, if all is good you may just have to replace it.
P.S. If the one you are buying from the junk yard cam out of an 86/87 model check for a strip of green tape on the bottom, a defect prompted a recall from a broken solder point, the green tape marks the ones the recall was done on.