I started my 91 Formula, LB9, 5-speed. First off, I had to press the gas pedal a very small amount to start the car. It held an idle at a fairly steady 400 rpm for about 10 minutes. I decided that I wanted it to idle higher so I tried to set the minimum air. I grounded B to A, turned the car to on, waited a minute, and unplugged the IAC. When I then started the car (again, I had to slightly press the gas pedal) to adjust minimum air, the car would not hold an idle. I extended the throttle stop screw with the car off. With the IAC hooked back up, the car idled at 500 rpm. I then set the timing (for the first time after reinstalling the distributor). I plugged the IAC connector back in. The timing was at 4* ATDC; I set it to 7* BTDC (EST was disconnected).

The car's idle speed was nice and steady at 700 for maybe half a minute, but it then began to oscillate from 500 rpm to 700 rpm with a period of close to two seconds. I still have to slightly press the gas pedal while cranking to start the engine. I did not check on the minimum air again, but that should not be the problem. I wish I had a scan tool, but I don't.

Why does the idle speed now oscillate and what do I do to steady it? If it matters, the only exhaust on the car at this time is SLP headers and header pipes.

 

I had a problem similer to what you have. Every time I went to start my 350 TPI i had to depress the gass peddle. I ended up changing the EGR valve and the problem went away. So I hope this helps you out. Let me know how it goes.

 

I started the car again tonight and it idled beautifully at 700 rpm. There was not the slightest hint of the earlier oscilation. I don't remember for sure if I had to press the gas pedal or not, but I would lean towards not.

Is having to press the gas pedal VERY slightly on startup a problem in itself? (i.e., if it doesn't hurt anything and it's not indicative of another problem, then I don't care about it.) I'm not setting any codes, if that helps.

EDIT: I disconnected the battery for a few hours before this. I'm really showing my ignorance, but could the car have idled nicely because I cleared the codes? What I meant above was that although I had no codes on startup, I also set no codes while the motor was running.

 

i have the exact same problem
i replaced my iac adjusted everything everyway i could and it still does it
i have know clue
i know cars but this problem i can figurout
maybe i could try changing my egr but i dont know
i think the problem might be that the computer is not holding the adjustment
maybe a computer problem
i ahve know clue
help me!!!!!

 

I driven the car since then a few times and there is no sign of a problem. Maybe clearing the codes "fixed" the problem. Like I said, above, I'm completely ignorant (for now) when it comes to the computer.

 

I remember reading somewhere that on some cars you have to disconnect the EST wire while setting the idle speed. It is the single brown wire in the harness on the passenger side, by the AC unit. (at least on my car it was...)