Captain Obvious

First, to answer the question... No. It is not doable without taking the dash out. If you didn't have A/C, you miiiiight be able to replace the foam on the visible side of the door, but even so, that's not the side that you're having trouble with. You're looking to deal with the side you CAN'T see. However... When your system is switched to A/C, all coolant to the heater core supposed to be shut off. Automatically. So if you're getting hot air out of the heat holes, the valve that is supposed to cut the hot coolant is leaking. If the air coming out is "ambient" (unheated and uncooled), then maybe it's just the foam on the mode door. But if it's hot, then you have more than one issue.

According to the 74 FSM (page EF-9), the electric fuel pump should deliver 1400 cc (85 cu in) in one minute or less. And also on the same page, it lists the pressure spec at 4.6 psi. No idea if that's what they actually did, but that's the spec.

Yup! There it is! Thanks for the pics! So that little hole partially blocking the return line of the rail is what provides the fuel pressure to the carbs. If that return line wasn't necked down like that, the pressure in the rail would be pretty much zero and the carbs wouldn't get any fuel. That orifice provides the restriction which generates the pressure in the rail.

Yes, there is an "on-purpose" restriction on the end of the return side of the fuel rail right where it transitions to the rubber hose (over by the fuel pump). It's a swaged down tip brazed onto the end of the return leg. I've got all kinds of pics of all kinds of carb stuff, and I can't find a single detailed pic of that orifice restrictor. So, all I've got is this: I'll take a look to see if I have any 240/260 fuel rail stuff around here. Not sure I do, but I'll look.

I'll see what I have for ECUs here. I don't know if I can tell anything from the label, but I'll look. So here's to hoping the lean boot thing just doesn't come back, but in reality what will probably happen is that it'll occur when you're dressed nice and don't want to go popping the hood and pulling wires off and all that. Haha!

Good. Glad that worked out. It's the little things!

It's not supposed to turn easy. They designed it so the threads are intentionally tight. They don't want that thing screwing in over time as you use the brakes. But it shouldn't be impossible to turn either.

Understood about the timing. Maybe you'll get lucky and it will never happen again. (Suuuuuuure). And I'm not proposing any theories yet... I'm just trying to come up with something, anything. to maybe point in the right direction. I'm thinking that if you do the "START with engine already running" a couple times and nothing changes, then maybe it REQUIRES the engine to actually come to a stop to reboot. Injectors need to stop injecting, coil needs to stop coiling... all that. On the other hand, if you do that test and the lean condition DOES go away, then there might be some info to be gleaned from the fact that the engine does not need to a complete stop in order to make the lean boot go away. Vibrations don't need to stop, starter does not need to suck a big hit of current from the battery. Alternator doesn't need to bootstrap. That kind of stuff. Just fishing at this point.

The engine should already be running. Let me try again to explain. Next time the car starts and you notice this lean-boot condition. Let it run, but while it's running, pop the hood, get out of the car, and disconnect the small black/yellow wire from the starter solenoid. Then get back in the car (all with the engine still running), and turn the key to START. The engine will already be running (because you never shut it off), but the starter will not engage since you have that wire pulled off the starter. Flick the key between ON and START and see if it drops out of lean boot and into regular running mode. Does that make sense? You can drive like that with the starter wire disconnected, but just remember that when you shut the car off, you'll need to reconnect the starter wire to get the car to start again.

OK. I had a couple goofy ideas. First thing... The original EFI systems were fully analog and there were no computers used. But then some time along the way (early eighties), they started putting embedded microprocessors in the ECU's and using software to do the control instead of analog voodoo. Why does that matter to me? Because if it's got a microprocessor doing the control, then this kind of latching behavior would be easier to explain. So I know your Z is a 78, which should be fully analog, but do you know for sure? Have you ever had the cover off the ECU? Got any pics? Second thing... There's a circuit inside the ECU that is supposed to provide a little extra richness boost immediately after starting the engine, and that extra richness gradually tapers off (30 seconds or so) after the key is released. Maybe that section of the circuit is not functioning correctly, or maybe that circuit is being fooled into thinking that the key is always in the START position. When you turn the key to START, a couple things are supposed to happen: The starter engages (duh) The floor temp lamp light is supposed to light up (lamp test for CA models only) The cold start injector gets the opportunity to spray (depends on how warm the thermotime switch is) The ECU get's the signal to begin the START enrichment boost So here's what I'm thinking... Next time the car lean boots, try this: 1) WHILE THE ENGINE IS RUNNING, pop the hood and disconnect the small Black/Yellow wire going to the starter. 2) Turn the key to START again with the engine still running in lean-boot. Don't worry... With the B/Y wire disconnected, the starter will not engage because you have removed it's signal to engage. But all the other "harmless" functions will be activated. The floor lamp should light up, the cold start injector should give a squirt (if it's cold enough), and the ECU should get a fresh "START" signal. The idea is that you can flick the key between ON and START a couple times to see what happens to your mixture readings. See if your lean boot symptom goes away without having to shut the car off completely? Maybe it'll go away with just a flick of the START signal. Like I said, I'm just throwing some stuff out there!

Gotcha. I'll think about it, and the meantime, let's hope someone else knows exactly what's going on!

Yeah, it unfortunately sounds like the bowl level is going too high for some reason. Either the fuel pump is forcing the valve open, or the setting was always ballpark and never precise. The wet set method is a bit of a pain, but it's the only way to be sure. If you want to confirm the theory, you can unscrew the air vent nipple on the top of the float bowls and see what happens. Nothing is supposed to happen, but I suspect you're going to have fuel come pushing out of those vents. And the only way that happens is if the bowl level is too high.

This is a 78, right? With mostly stock EFI system?

Crap. That's the wrong answer. Haha!! I was kinda hoping you re-booted it right away and it went back to normal. I think that would be easier to figure out. (Not easy, but easier? Maybe?) So I got nothing right now... I'll think about it and see what I can come up with? Or maybe someone will chime in with some uber-smart theories? Not temp related... Not vibration related... Not system voltage related......... I wonder if my thinking cap will fit under my aluminum foil deflector beanie.

Hmmm. Interesting. And no idea why that would happen. So have you driven the car with this "lean boot" condition? Does it stay lean the entire time? Or did you shut it off and restart as soon as you saw the issue?

Haha!! Well however the math works out, Wally, good luck with the cleanup and don't brush so furiously that you wear through the seats!!!

That will show up on a clear tube wet test if he uses the fuel pump to fill the bowls.

No. You should assume that your .dry 55" measurement got you (hopefully) close to your target, and do a REAL fuel level test using the clear tube method. The mechanical .55 is just to get you in the ballpark. Sometimes it works straight out of the box without subsequent adjustments, and sometimes it doesn't. You need to verify the level on the car with fuel in the bowl, not dry on the bench.

I'm thinking if they are 50% better, he needs to the same amount of brushing again and then they'll be 100% better, right? Math.

I think there is some wood in there, but put it this way... Nobody should worry about termites.

I'm having a hard time reconciling these two statements: About the only way to get fuel to pour out of the carb mouth is for the float bowls to overflow. So if the float levels are withing spec, I'm having trouble correlating that to fuel coming out of the front of the carbs. Are you saying that you have checked the bowl levels right WHEN the problem is occurring and they are in spec? Even while there is gas pouring out of the carb mouths?

Me too.

Not sure about the 240, but the 260-280... Easy-peasy:

Exactly. It's getting normal lots of places. That's the vaccine at work. Every person who gets the vaccine makes things better for all of us. But it's no time to let up. We're not at herd immunity yet, and mutations are still a risk to everyone. Even the people who have gotten a vaccine.

Stud!! I got that part right! Wrong hair color though.