Captain Obvious

Oh, then forget about what I said about coming down off load to idle. That's not it. You had mentioned earlier that you thought it would be ignition related since it didn't stumble before it shut off. In my experience, there isn't any significant stumbling when the fuel pump shuts off. It just dies. So how about this (since you have a 78)... Your oil pressure drooped enough in your hot idling engine and opened up the fuel pump switch and shut off the fuel pump? Or the "L" terminal on your alternator is acting up at low RPM's. That could also shut off the fuel pump.

Bummer. Was hoping it was just an instrumentation issue. We're back to where we were before. Keep us posted!

I would start by watching for the situation to occur again and try to get a feel for what's going on. I think all this poking around with the ECU as a culprit is premature. My car would sometimes shut off when I pulled up to a stop. Engines don't like a rapid drop from load to idle. It's just unstable. Modern engines running a fast control loop use technology and speed of response to take care of it, but back in the old days, it was never uncommon to pull up to a stop sign and sometimes have your car stall. If your car was tuned properly and running good, it was "rare", but it did happen. I think mine was caused by a rich fuel mixture, a BCDD that was disabled, and an idle that was set a few RPM lower than spec.

OK, so that's an original 78 ECU and there's no software in it. In other words... I don't think there's anything inside that can that would cause a lean-boot that would last the length of the drive and then be reset by a reboot. Still sounds like a software issue to me. And with that in mind... Something we haven't discussed in depth... Jonbill had asked earlier "when it is in this "lean" state, does it feel normal to drive, or hesitant?" Your A/F meter is the only entity attached to the car that has any software in it. Let's consider the prospect that it's a measurement phenomenon and not "really" occurring? So can you tell from driving the care while it's in the lean-boot state... Does it feel lean, or does it feel the same as when it's not measuring lean?

Forgot to ask. There are some things on that diagram that might not really make sense to some people... Do you know what means when there is a fuse for "defogger switch" and a completely separate different fuse for "defogger relay"? Does that make sense? (Just in case it didn't make sense) The "defogger switch" is the low current side. It energizes a relay (the "defogger relay"). The current going through that fuse does actually ever get to the defogger itself. The fuse feeding the defogger relay is the high current side and provides the current that actually flows through the defogger. But the tricky part is... If EITHER of those fuses is burned out, the defogger won't work. Does any of that make sense at all? LOL. I also don't like "steering lock switch", but that came right from the documentation. That is the "will activate the beeper if the key is in the slot while the driver's door is open". But I ran out of room. Haha!

Not pretty, but I'm hoping it's accurate. I'll give it some time to cure and then I might spend a little time making it look prettier. And if I get a chance, maybe I'll look at some 76 or earlier. I don't know much about the 240's, but I believe everything 260 and 280 had both lighter and glove box lamps. Maybe your bulbs are just burned out?

I traced a couple more fuses. Note that this has not been checked by anyone else, but here's the latest. Also note this is for 77 and 78. I haven't looked into any years earlier than that:

That connection to the 77 voltage regulator is the "IG" terminal and I don't remember the details of that what that connection does. I converted my 77 over to the 78 style internally regulated system a couple years ago, and as part of that conversion, I purged my brain of the finer details of exactly how the external voltage regulator works. I used to know, but now that I don't need to know anymore... Most of it left. So, having a 78, you don't really need to know either.

I traced a couple more fuses. It's not pretty, but this is just what I have so far. I'm not going to bother making it pretty until it's been checked by other people. This is for 77 and 78 (with differences between the years noted if there are any): Hopefully someone who is bored will check my work?

Ever notice those weird holes at the bottom of the fuse box? : I think there might be an obscure mention of it in one of the FSMs. That said... If you know you're blowing fuses, you might want to carry more than just three spares.

Well let's hope tit was an infant failure on one of the 2017 fuses and not a real issue. You'll find out at the most inopportune time. Do you have the spare fuse holders populated in your fuse box?

Yeah, it's always unnerving when a fuse blows. You put a new one in there and wonder how long it's going to be before it happens again. So was the fuse that blew a new one? Or do you think it was from 78? The reason I ask is that if it was an old one from 78, I wouldn't' worry too much about it. In fact, I would replace all the fuses with new ones. Time probably hasn't been kind to them.

I took a look at the 78 wiring diagram, and the fuse that powers the voltmeter powers the following: Hazard flasher Steering lock switch (which then feeds the left hand door switch, which then feeds the door open warning chime) Voltmeter needle For 78, that's it. Just those three things. So since I know that the top two fuses in the left hand column are for right hand and left hand headlights respectively, we have three of the fuses sussed out (for 77 and 78 at least) Someone else can take a couple fuses, and pretty soon, we'll have them all done!

I've not seen a master list like that. And I agree... I think that would be a valuable tool. Kinda surprised someone hasn't put one of those together. Of course, it would be year specific.

Haha!!! Yessir!! I think everyone can agree that a well formed nubbie is a thing of beauty!

Yeah. the 72 carbs, even though they are mostly like the previous years, they do have their weird idiosyncrasies. I find it neat to see some of the changes put into play on the 72 3-screw carbs that were clearly stepping stones to the flat tops they used the next year.

Oh, and all that about dome alignment... Here's my dome / nozzle sleeve alignment tool. Centerless ground precision stock. Sub-thousandth tolerance. Take out the suction piston and put this in it's place: So how do you use it on the 3-screw round tops? Well, if you still have alignment nubbies, then you don't need to align anything. It should already be aligned. But if one or both of your alignment nubbies are broken off... Insert the alignment tool into the carb body and put the dome on: And then while the dome is held in "perfect" alignment, you cast the alignment nubbies in place: Fill up the hole and then wipe off any excess: Then once the nubbies cure, pop the dome off, and presto... New nubbies! : The eagle eye might notice that carb is a flat top, but the 3-screws used the exact same alignment technique.

I respectfully disagree. That needle/nozzle alignment method will not work on the odd bird 72 3-screw round tops. The nozzle sleeve is not adjustable on the 3 screw round tops. On those 72 carbs, the nozzle sleeve is pressed into the underside of the carb body and does not come out. Once it is pressed into place, it's fixed. No adjustment: So since the nozzle sleeve is hard fixed, they do the needle alignment using the dome up top instead. The three hold down screw holes are intentionally sloppy allowing the dome to be positioned in the correct location. With that in mind, the needle alignment on the 3-screw round tops is set at the factory using the cone shaped alignment nubbies. Cast in place once the domes were aligned with the nozzle position. The alignment was set at the factory. Once and done and (assuming the original nubbies have not been broken off) nonadjustable after they leave the factory. I assume they took that power out of the owner's and dealers hands because they felt they weren't doing it right. Here's a 3-screw nubbie:

Glad to help. Sorry I didn't have better news or a simpler way to fix it. And since I'm already the bringer of bad news... The normal temperature control lever should also be able to block off the flow to the heater core. If you've got it slid all the way to COLD, then the heater core should be cut-off. Yes, when you switch to A/C, the other valve should do that automatically, but you should still be able to do it manually with the lever actuated valve. So if you've already got the temperature lever slammed all the way to COLD and you're still getting a hot air through the heater core, then you've actually got three problems: Your mode door foam is leaky. Your vacuum-cokk valve is leaky. Your temperature control valve is leaky. All pretty much normal for these cars.

And by the way, I think we're all saying the same thing here... Carbs need fuel in the bowl in order to operate correctly. And that fuel won't get into the bowl by magic. It takes pressure. It might not take much... In fact, in many circumstances, simple gravity is enough. But there must be something that pushes fuel into the bowl. But for a car like the Z with a tank located below the level of the carbs, you need a pump. And a pump needs specs. So Datsun provided some specs in order to qualify and test the pump(s) used in the cars. "Should be able to provide this volume at this pressure and no higher than this." It can be somewhat arbitrary as long as there's enough fuel provided at extended full throttle, but not so much as to force the float valve open at idle when you don't need nearly as much fuel.

Cracking pressure is a term most associated with check valves. They are be held closed by something (usually a spring), and there is a certain amount of pressure required to overcome the combination of stiction and the force of the spring in order to force fluid through the valve. And even though not necessarily considered a check valve, the construction of the needle valve is identical to a check valve. The only difference is that the compression of the spring (and hence the force supplied) is variable because of the moving float. Here's some good pics and description of cracking pressure: https://www.hydraulicspneumatics.com/technologies/hydraulic-valves/article/21884995/engineering-essentials-pressurecontrol-valves And from that page, here's a neat sketch showing why (even though it's not a "check valve", the concepts cannot be ignored even for the float valve. If you replace the adjustment screw with the float tang, here's the carb float valve:

Yeah, I took a look at the vacuum diagrams and it's not a vacuum leak. The no-vacuum "default" position of the water valve is closed and the default position of the mode door is "dash vents". In other words... If there is no vacuum applied to anything, you get recirculated air, unheated, out the dash vents. If you add evaporating refrigerant, you get cold recirc air out the dash. So back to the original issue... It sounds like your water valve is passing water when it shouldn't be and your mode door isn't a perfect seal. No you can't refoam the mode door in-situ, and working on that water valve up under the dash is "possible", but very uncomfortable without pulling the dash. You could install a manual water valve up in the engine compartment. Being in Dallas, I gotta believe you could use a manual valve to completely cut off all the water to the heater core for most months of the year, and you wouldn't miss it. Wouldn't "fix" either issue, but it could be a relatively simple workaround for the majority of each year.

I think if your hands are small and you are a limber gymnast (who have small hands anyway, so I guess the crack about the hands is redundant) I think you can replace that valve without pulling the dash. It's behind the blower motor tucked up where the heater core hoses are. It's a bundle of snakes down there. However, first thing I would look into is... Is that valve getting it's signal to close? In other words, just because that valve is passing coolant even when it's not supposed to, is the problem actually in the valve, or is the problem somewhere else (like a dry rotted vacuum line under the dash)?

It's not the weeds at all. You need pressure. Enough to overcome the cracking pressure when the valves are closed, and enough to overcome the orifice restriction that is the needle valve. Tables and tables and charts and formulas for the pressure drop across an orifice. Reynolds number, temperature, flow rate, viscosity..... Depends on a whole bunch of factors, but the bottom line is you need enough pressure to push the fuel in under all conditions, but not so much as to force the valves open when you don't want them to be open. It's a little mini-control loop with pressure being the energy that makes it all work. If it worked without pressure, it would be a perpetual motion machine.

Carbs DO rely on pressure. You need to overcome the cracking pressure of the float valves. It might not be a lot of pressure required, but it's non-zero, and the system will not work without it. Too much is a problem, But no pressure is a problem as well.