Honestly, I was hoping to find a failed timing belt when I opened it up yesterday. That would have been an easy explanation to this mess! Instead, the timing belt was in good shape and there was no indication that it had slipped, as all of the marks lined up. I went ahead and replaced the timing belt while I had it open.
I have seen in other threads that a blue spark is preferred, but I have also seen a differing opinion stated that a yellow/orange spark from a plug is also fine. Does spark color really matter? Besides, I have replaced just about everything in the ignition system except for the Hall effect sensor and the ECU.
What really confuses me is the fact that it started and drove fine on Sunday morning (1.7 miles, a short trip) but wouldn't start just two hours later. In that time, weather conditions were unchanged, so no rain or other factors are suspect.
This does remind me of an incident with a 740 we had four years ago that refused to start. After checking everything under the sun, I finally gave up and put it in the shop. After much searching and testing (and $600 later), the mechanic found a wire leading to the in-tank pump that had been rubbed bare against the trunk floor from the luggage we had hauled on a recent vacation. I don't think that is the problem now since the plugs are getting fuel on them, but it does point to a single wire as a possibility, which I hate to admit. Are there any other possibilities before I start trying to trace wires all over this car?