There was a solid clue during setup...when sliding the connectors into the harness...they are whisper-tight. Meaning: Not at all. Solid money is on worn out/loose harness connectors but we'll see if banging on ECU does anything after the normal "give up and die" timeframes of 20-40 mins.

RESULT: Loose harness connector(s). Now to figure out how to either tighten/tweak the harness connectors...or replace them 🫤

There is still a mystery (to me, at any rate) on why the engine would sometimes simply shut off like a switch and sometimes stutter-stop, and why sometimes it would re-start immediately and othertimes re-start 10-20 mins later. My hypothesis is that by simply getting in/out of the car while diagnosing I was re-wiggling the loose connector(s) such that they had good-enough connections, and the changing fault symptoms were a result of different pins or a different combo of pins losing contact.

The end result is the same: Fix all the connections and do pin-drag tests on all of them to make sure they are sufficiently grabby.

Thank you to everyone. I hope it was fun for you too.

Don't overlook that when you have the connector disconnected from the board that the solder joints are probably less-stressed. I didn't see the common cracked solder joint problem mentioned in the thread.

For some reason the internet is not picking up the old threads when I search. I only came across one. I've seen the others though.

2 hours ago, f1d094 said:

RESULT: Loose harness connector(s). Now to figure out how to either tighten/tweak the harness connectors...or replace them

Glad you found something. Are you sure you have identified the root issue there? Reproducible at will? Double dog sure?

And as for the mystery about behavior, electrical intermittent connections are like that. Barely making connection by a molecule. Does unpredictable stuff.

Oh, and it's been many moons since I've looked at that connector on the end of the EFI harness. I assume the terminal contacts inside are replaceable? Have you looked into that?

For the record, I just read that zcar.com thread and see that I might have actually borrowed the transistor replacement idea from ZXPastor. Not sure. I got the replacement transistors at a Fry's Electronics store before they shut down.

Cracked solder joints aren't uncommon on the old Datsun electricals. The headlight and running light connections, on the top of the steering column, crack and break also. You might add some relays before those fail. Full lighting circuit current runs through them. They get hot.

Will be following to see how things work out. You'll have three ECUs to swap in and out for testing and comparison. Good times.

10 hours ago, Captain Obvious said:

Are you sure you have identified the root issue there? Reproducible at will? Double dog sure?

Not 111% certain on root cause yet, but 100% certain it isn't the ECU (cold solder joints or otherwise). The problem was very reproduceable previously by hitting the ECU hard enough at any time, not just when warm. During testing, I BEAT on the ECU with zero effect. What I did not do is whack the actual harness plug as planned, the pins simply were not secure enough. They did not slide out on their own, but any brushing of the wires in-situ would likely have caused them to slide out or lose contact. They were simply not tight at all. If you look in the photos closely you will see I have a rubberized tie-wrap wudged between the dual-bank wires to help them maintain contact. I don't have an official "pin drag" tool, nor do I have any pins that are the precise size of the ECU pins, except on my spare ECU...I tried a bit of googling about Bosch 35pin connectors to no avail but may revisit that. I should note though that the blade connectors I used are definitely thinner than those on the ECU. I had to really really push to get the female ends of the extensions on the ECU vs the harness connectors that could have been pushed in by an ambitious ant.

At this point I have removed the 3 miles of electrical tape wrap that was over the rubber harness covering all the way down to the harness head. I did not have enough time to figure out how the harness head disassembles, so if anyone has one laying around and wants to give me some pointers that would be great. I've not had any time to give it a close look yet. The best I found was one for sale on EBay that has some useful photos. I have a large array of spudgers and other disassembly tools, so I'm sure I can get it apart, but it is always nice to have a look at the inside of what you are working on first. Old plastic is old.

My biggest concern is that the problem might be an internal break in one of the wires that isn't discernable externally. Hopefully not though, all of the wires under the tape/rubber look like they were installed yesterday; they are that fresh and new. There is even some marking paint on them that is still pliable; mummies were less well preserved before burial. Another concern I have is when I get the casing open that all the individual connectors come flying out, leaving me to do wire-tracing to re-identify their homes. Hopefully they have individualized stays that are still in good shape.

I don't know how I will tighten up / micro-crimp the female leads or if I will simply replace them. There isn't a lot of play in the harness so I am loathe to cut them shorter to get new ends on. I don't even know what these types of ends are called or if they even have a special name. I think they are the same as an injector plug and I have several spares of those, so next weekend I'll look into that.

The adventure continues...

9 hours ago, Zed Head said:

Cracked solder joints aren't uncommon on the old Datsun electricals. The headlight and running light connections, on the top of the steering column, crack and break also. You might add some relays before those fail. Full lighting circuit current runs through them. They get hot.

I'm generally familiar with the brown-out condition the Z's ran under due to an undersized alternator or so it was explained to me when I replaced my headlight switch assembly... I addressed that a couple decades ago by installing a high output one. Also, during the pandemic, one of my projects was to convert the entire car to LEDs (minus the headlights), which dropped about 200W of load off the electrical. The headlights I was planning on doing very soon...but now that is clearly on hold until I have a drivable car again. ;)

Edited 3 hours ago3 hr by f1d094

I think you would be surprised at the low amount of insertion force required to mate one of those female contacts onto the male tab. They do that on purpose because by the time you get to 35 of them, it adds up.

So if you aren't 111% sure that you have a contact issue, I might keep investigating.

But if you are looking to replace some contacts, i believe everything involved is old Amp (became TE) "Junior Timer" series. JT.

There are many different categories within that family, but I believe the contacts used on the harness side are specially designed to be a lower insertion force. And yes... there should be a retainer tab on each contact. You should not have to worry about all the contacts come flipping out by accident.

I think. ☺️

Wow. That was fast. You should play Z-Car Jeopardy and rake in millions.

Haha! I actually fade in and out of consciousness on the forum. You just happen to catch me at a time when I'm actually on-line and logged in.

I spend significantly less time here than I did before the last couple software upgrades. Hope that trend does not continue.

3 minutes ago, Captain Obvious said:

I might keep investigating

I'm right there with you. Hence the disassembly of the harness and connector head.

I am very very confident at this juncture that it is something in the head itself...so either a damaged connector, or worn out connector, or bad wire-to-connector crimp/solder. It has to be one of these things. There just isn't anythinge else in the car that would be affected by my tapping on the ECU.

"When you have eliminated all which is impossible, then whatever remains, however improbable, must be the truth."- Sherlock Holmes

Wiggling the harness plug didn't do anything, almost certainly because it is so securely held in place to the ECU, and the ECU to the body. Tapping directly on the harness plug however had the most immediate and obvious effects. This is what made me think it was the "choke coils", their proximity to the most delicate "tap point", but that has now been elimiated. This leaves me with a "I'd bet $1000 USD" confidence level on it being something to do with the connector internals...and so I'm going to disassemble it and have a look. Since the pin receivers are where all the action is, I would think they would be the most likely culprit. I've unplugged the ECU only a handful of times in the 25 years I've owned the car...but who knows what the PO may have done.

Now I'm all fired up. Maybe I'll do some re-arranging and find time during my evenings this week...

Edited 1 hour ago1 hr by f1d094