Here's today's update. I worked on the Bronco all day - started with checking the codes once more. I had one, in the KOEO mode, stored: number 18. Book says that's " IDM circuit failure or SPOUT circuit grounded". I found an interesting thread on this here, the original poster described exactly my issue (random stalls):
www.fullsizebronco.com
I did the tests that I could understand (much of this thread goes straight over my head, I have almost no experience troubleshooting wiring issues, much less on this era truck):
- tested the spout connector itself
- tested for faulty ground on both spout connector wires - nothing (both had in the many millions ohms resistance, as I understand they should have)
- tested the resistance of one wire (forget now which) that was supposed to have between 20-25K ohms resistance, due to an in-line resistor 'somewhere' in the wiring loom under the hood (a noise suppression resistor, going to or from the coil I believe) - it has 22.5K ohm resistance - so that seems good
- I replaced the not-that-old coil with a new Motorcraft one I picked up yesterday
- I removed the distributor, inspected it, and placed a new TFI module in place of the still-pretty-new Motorcraft one, just for good measure. Re-installed, set timing (with spout connector removed) to 10 degrees advance, and re-installed the spout connector
- I also tested two of the wires going from the TFI to the ECU/computer for continuity - green-yellow ones. They tested out fine but, in the process of identifying the correct wires at the TFI connector, I discovered that a previous owner/mechanic had already replaced the original TFI connector with an aftermarket/different one, since all the wires coming from it were black. I traced the wires to where they had been fused to the original wiring loom and tested each of those connections for continuity, wiggling each one in the process to try and uncover a break in any one of the connections. No luck, they had been done pretty well, with shrink wrap over the joins and that all covered with electrical tape. I did notice that one of the original loom harnesses that about 3 wires from the TFI go into (the TFI connector separates into two bundles of 3 or so wires each, going in a different place in the wiring looms) was missing the last section of 'wire sheathing' (sort of a thin metal foil probably designed to keep out radio interference to the signals going through the wires). I did not have anything to replace it with at hand so left it as-is (after all the truck has been running fine without this random stall problem for almost 4 years I've owned it)
After all that, drove home at the end of the day, all good, until the engine was warmed up well and now I have a different problem: rather than cutting out and stalling altogether, it just misfires bad for a split second: I can see the RPM gauge drop to almost zero and hear the truck stalling, but then it 'catches' itself, tach needle goes back up to where it was and it just goes on running. As I was getting closer to home, it started doing this more and more often - but it never stalled completely, as it did before.
So, definitely ignition related and it also seems highly likely heat-related. I know, the TFI module is sensitive to heat and I could mount it remotely etc, but this is a brand new TFI module, replacing another brand new one. What is at least somewhat encouraging is that, whatever I did (and I manipulated a lot of wires, unplugged and plugged back in a number of connectors etc), it has altered the symptoms at least somewhat. So the 'code 18' is probably the clue to my mystery.
What I now want to do is find someone local to me (Los Angeles, lots of shops here) who is at least somewhat experienced with diagnosing and addressing electrical / wiring / harness / ignition issues such as this one. I spent all day, after having spent so many hours before, and am just out of energy to keep at it. I am not experienced in this type of issue and when I read various forum threads on this 'code 18 issue' it became clear to me that it is not uncommon, and there ARE people with a lot of knowledge and experience and could likely resolve it much more efficiently than I can. I really don't want to sell it in this condition, or junk it, it's too nice for that.