Just get in it, set the parking brake hard, step on the foot brake, restart the engine and then immediately put it in gear and
feather the gas pedal up to about 1000 RPM and hold it there for a minute or so. (ONLY FEATHER THE GAS IF YOU HAVE TO)
It should automaticaly go to IN GEAR mode for rpm which opens the IAC more anyway but if it doesn't want to then idle it up manually before it dies and slowly come off it as it picks up itself.
You have to put it in gear and up the rpm within 3-5 seconds if I remember right or this doesn't work.
Keep it in gear for at least 10 minutes idling with the AC off and not doing anything else. It should relearn fairly quick but 10 minutes is minimum. Then drive it around the neighborhood so it can relearn everything else. It will probably run weird for 20-30 miles or so.
If the battery went completely dead this is probably all it is. It happens sometimes.
All 91's that I've ever seen are EEC-4 computers. This is the standard way to fast learn the idle again after a dead battery or a computer swap.
It should pick it up itself but it probably has a filthy IAC solonoid. If you have the IAC that has two screws attaching the solonoid to the actual valve you can take it off the throttle body first, then seperate the solonoid from the valve and spray some foamy window cleaner in the valve then let it sit a couple minutes. Then rinse good with hot water. Repeat this untill it's clean.
DO NOT use carb or brake cleaner. Unless you want to be cleaning it a lot more often. It will take off the anti stick coating on the inside of it.
eec-4 obd-1 and obd-2 will all set codes when the engine is first started and cold. Not always though. Usually when it doesn't on an eec4 then it's just a failure to control idle problem because it's memory has dumped and needs to relearn. It doesn't know how much to tell the iac to open till it's too late. This rarely sets a code because it tried but the engine died before it got there. Now if there were an electronic fault then it will set a code.
