Up & down idle 88 Duke when warm

[GTRS Fiero]

I hope the issue is not the ECM itself.  Does the car run fine, hot, above idle?

[Donster]

Runs like a charm, even good gas mileage when hot, just won't idle good.

[GTRS Fiero]

My understanding is that the ECU controls idle after the engine is warm, below a particular RPM.

[Raydar]

The ECU always controls the idle. Just in closed loop (O2 sensor warmed up and functioning) it's got more to think about. (Various sensor readings, etc.)

[GTRS Fiero]

That makes sense, but something is amiss.

[Donster]

Could the O2 sensor be the culprit? Haven't checked it yet. How do you best test it?

\D

[GTRS Fiero]

Could the O2 sensor be the culprit? Haven't checked it yet. How do you best test it?

\D

I think the MAP and O2 sensors could also be suspects.

Sure could.

[GTRS Fiero]

I'm doubting the ignition coils, and you replaced the intake gasket.  O2 sensor?

New IAC does not equal good IAC.

It's been suggested once or twice.

Usually, a bad O2 results in smelly exhaust.

[Donster]

So how does one test O2 sensor?

\D

[Fierofool]

That's a good one.  It's not a temperature sensor, so the chart wouldn't work for it.

[Raydar]

So how does one test O2 sensor?

\D

Once the engine is warmed up, unplug it. It will set the O2 sensor code (pretty much guaranteed) and should turn on the CEL light.
But if the idle settles down, the ECM would seem to be fighting with what the O2 sensor is feeding it.
If that's the case, I would be inclined to replace it. No guarantees, but what else ya' gonna do?
I don't think it's the coils or associated pieces.

Edit - If you've got a scan tool, it should show ~.445 volts (DC) when the O2 is cold. (That value is "plugged in" to the PCM, and is used as a default, until the O2 warms up and starts responding to actual exhaust readings.)
Once it warms up, it should bounce back and forth between .1 and .8 volts (or thereabouts.)  It should spend about half the time on either side of .445 volts.
If it parks on one side or the other, it thinks it's seeing a continuous lean or rich condition. Towards .8 is rich. Towards .1 is lean.  If you are accelerating or decelerating, a temporarily full rich or full lean condition is normal.
The O2 CAN lie. At one point, the one on my 4.9 indicated full lean (very low voltage.) The PCM compensated by dumping in as much fuel as it could. Failed a smog test horribly. Also killed my catalyst. My gas mileage was also crap.

[Fierofool]

Don, take a look at the thread I just started about the MAP sensor.  Might have some answers for you. 

[Donster]

Thanks Charlie.........good reading.
BTW: Installed new O2 sensor today, ran the car hot, no change, idle still hunting. 
\D

[Raydar]

Thanks Charlie.........good reading.
BTW: Installed new O2 sensor today, ran the car hot, no change, idle still hunting. 
\D

GAAAAHHHH!!

[GTRS Fiero]

We need to see that the computer is showing, whether voltage, or sensors.