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Hi
Idle speed and roughness issues usually relate to an air flow disturbance up setting the mixture balance to all the cylinders.
There can always be one cylinder that is a little bit more sensitive to conditions. Not all the cylinders are going to exactly the same due compression wear or valve deposits either. It's life with its twists.
Since it appears to be sporadic. I would look at things that can be changed from air leakages or signal drop out.
The Throttle Position Switch is the first one to come to mind. It fits in between what the throttle plate does and it's how the Idle Control Valve gets told to respond.
You want the IAC to control all of the idle air input. It has a limited range of flow.
Make sure the throttle plate is absolutely closed in the bore and not being held open by that silly stop screw. It gets adjusted to high in most cases.
It is a stop screw only! I barely give it a quarter turn or less!
Make sure the switch is set properly by using an ohmmeter. Look for a good repetitive continuities. Responses every time the throttle opens and closes.
This can be checked at the ECU on certain pins it's voltages to check out wiring harnesses on these mid eighties cars but you will need more information from the gurus on the board.
The Idle program is different from going OFF idle or onto power on/running. The Electronics have to get in time with transitioning mechanical mixtures. The new or good 02 sensor helps and you changed that.
The FPR works in here too. Tied in with vacuum signals and is independent of the ECU. You say the engine is fine there and the Mileage is good too! So, I would pass on it.
If it starts good from an eight hour stop over it's not leaking down either.
The IAC is working but might be hanging or lagging. The vane could be dirty or it has worn some brush grooves into the armature from many years of use. They get tapered and narrow in width and lose contact from excessive carbon dust. If it's over 100-150 K it's possible depending on how much idling did over its time under the hood. Another of life's variables.
Air leakages have to include the accordion hose for cracks along with vacuum hoses with splits on the very ends. I find them more on the under side where you can not see them! Another one of life's twist. The flame trap hoses, becoming swollen from exposure to oil vapor will loosen, of which, can mess with air input.
Do you have any idea how old the intake manifold gasket is or injector rubber seals?
Both are very reliable but can age.
Darn life and gray hair, if you got it!
(:-)
Phil
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