I've noticed that the idle does NOT increase when I turn the A/C on; this, however, could be due to the A/C compressor not turning on (possibly due to low freon charge?). The A/C has never been cold in the five years we've owned the car. The idle drops down slightly (about 50 rpm) when you turn the steering while while the car is stationary (I know that puts a lot of stress on the steering system, but just did it briefly to see if the ECM would respond).
IAC valve is a new, OEM part. Throttle cable is properly adjusted and does not seem to be binding. Throttle body has been cleaned. Throttle plate does not seem to be hanging up.
As for data logging, I'm not that technically (computer) saavy, although if I don't resolve this problem soon, I suppose I will have to learn to do that.
My apologies for scattering threads; I can see how it can be confusing. Even I'm forgetting how many I started.
I plan to do further testing for vacuum leaks. If I remove the fuel pipe protectors, it will give me more access to the bottom edges of the intake manifold runners, and possibly some of the tubing under the manifold as well. Instead of a smoke test, I am going to do a propane test, and make a wand out of 1/4" copper tubing that I can bend to help me get under the legs of the intake manifold.
As for the wiring: Yes, you are correct, the engine was replaced with a used engine in November of last year. Had an issue with a pushed-back pin in connector to the right-front O2 sensor and got code related to O2 sensor heater circuit, but I pushed that pin back and have not had that code since.
Recently, I tested for voltage, continuity and resistance between the ECM and sensors in question, and the wiring seemed to check out. More recently, I took apart and cleaned (with QD electrical contact cleaner) the engine/bulkhead wiring harness connectors, as well as the connectors at the sensors. Although I did not see any bent/broken pins, or corrosion, cleaning and reassembling the connectors made a definite improvement, lowering my idle speeds by 50 to 100 rpm, both hot and cold engine, so that makes me question if this is a wiring issue, instead of a vacuum leak. I have not tried wiggled the connectors, though, while the idle speed is surging, to see if that makes the engine act normally.
Or, I could have both a vacuum leak and electrical issue .... ggrrrrr.