1995 4.0 intermittent rough idle/o2 sensor observation

My 1995 ranger with a 4.0 seems to have an intermittent rough idle. It will barely idle in this condition, 400-600 RPM, sometimes stalling.

It usually happens when warm. Sometimes/often snapping the throttle seems to help.

I did recently swap IACs with one out of a 3.0 (my old ranger) which upon writing this post might be suspect (I had another spare one I swapped for this one awhile back after suspecting a different issue). I bought the truck a month ago and it was sometimes idling way too high, and I found a dead spot in the existing IAC. Being lazy, I grabbed this IAC out of my spare pile rather than pulling from the dead truck. I still have the wrecked remains of the old ranger (not yet sold for scrap), so maybe I should pull the IAC off of that and swap again?

That is assuming that the 3.0 IAC is compatible; it looked a bit different but the connectors, bolt holes, and mating surfaces all matched up. After the first swap, my idle hasn't ever gone that high (2000RPM) but still runs a bit high when not rough (1000-1200RPM)

I did clean the MAF with MAF cleaner and the throttle body with throttle cleaner (the later was quite dirty), but to no avial.

Another thing I've noticed tonight was something odd with the downstream O2 sensor. Just for my information I was plotting the 02 sensor voltages using Torque Pro, and it seems that while the upstream sensors are behaving as expected (switching between .2 and .9 volts), the down stream isn't switching at all. It was reading dead 0 at a somewhat warm idle, but after driving it was fluctuating between .7 and .2 volts, but very slowly (unlike the upstream sensors). Since it's the downstream sensor, I doubt that's the cause of my rough idle, but could that indicate a separate issue?

Any advice on either issue? If nothing else this weekend I will try swapping IACs with a known good one (assuming that using a 3.0 IAC isn't actually an issue).

Swapping IAC's from vehicles with different motors? Not sure on that one. I know one time, for shits and grins, and for the info, I tried taking the perfectly functional IAC off my Lightning and putting it on the Ranger. Same with the IAC from the Ranger, putting it on the Lightning. Neither would even start. Put them back on their proper vehicles and all was right with the world.

I thought at first, the only difference was the clocking of the plugs. Must be something else internally. Resistor, capacitor, different motor?? Hell if I know. But it didn't work for me.

I thought either the upstream or the downstream O2 sensors, one or the other, were turned off during warm up, so the PCM could richen the mixture for starting. Whichever it is, they don't turn on until the PCM switches from Open Loop (cold) to Closed Loop (warm), which occurs when the Coolant Temperature Sensor tells the PCM the engine is at full operating temperature.

In any event, my 3.0 Ranger is 19 years old, and during recent diagnostics, the O2's and everything else were still good. Badly plugged injectors were my problem, and not sure why I have such a hard time keeping them clean. PITA.

Did it again for a lukewarm start. O2 sensors (all three) 0v, running in open loop. Once the sensors kicked in, it went into closed loop and my idle stabilized.

Also while in open loop my fuel trim soared to 40% or so.

Any of this indicative of an issue?