Hi, I just got my convertible 305 TPI trans/am and I took it to the shop to get a rattle checked out. Got it back and my idle was all Fubared.

Now the car idles from 500 and a bit under to 800RPM.
It sounds like the engine drops then revvs up and drops and revvs up and drop and revvs back up(this happens idling in drive)
It will do this about 7 times before it stabilizes and it idles at around 600-650.
In park it will keep on doing this but there is a bit more time between the drop in RPM's/
Now, we took out the IAC and replaced it with a new one. Cleaned out the whole IAC housing (it was pretty dirty). However, that did not seem to help. I put in a set of Borg Warner replacement cap and rotor and I followed all the wires.
(Some of them were resting on the exhaust, so that might be the problem). At night it did check the car for arcing, but I could not find anything.

What would cause the idle to go up and down like that???

The engine is a Jasper replacement engine with 21K on it.

 

Kinda sounds like your timing advance is playing tricks with ya.

 

Well, this morning I was getting check engine light. It would come on, and off. I am yet to check the code, but after this my car idles fine .

Off to check the code!

 

Ok.
Code 33 and 42. The 42 code I got from unplugging the EST connector to set the timing. Code 33 is high voltage (low vacum) at the maf, or map sensor.
Not sure since this car is 1992. I think its a speed density so I am not too failiar with it.
(TBI guy here )

 

Oh, my car worked fine after it threw those codes. !?!?!