The IAC was tested as being good pre-install using a handy dandy power supply kept near my desk. Used a multimeter to test resistance, and all was in spec on both new and old.

Adjusted the idle speed as best I could, but there was no drop below that, leading me to suspect the newer IAC wasn't closing fully.

The wiring insulation certainly has potential, too. The Rochester winter has raised so much electrical hell. More specifically, the salt that they just about paint onto the roads has caused all sorts of corrosion in every possible region. I've found that it's in my best interest to regularly clean most of the frontal-region connectors as best I can on a regular basis, lest I suffer problems related to funky grounds or otherwise weak circuits. I certainly hope it's not the harness, as that's just a bit more time and money than I feel I can put into it.

As far as the injector wiring, using a stethoscope, I was able to determine that each injector was firing, so it's not that.
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1980 240DL, 1984 240DL, 1988 740GLE, 1993 850 GLT