Hey, funny story. I forgot I still had the rear on jack stands, and realized I'd only put in a few drops of gas. Maybe it wasn't getting any fuel. So I dropped the car, put in 2 gallons, and fired it up.
At least it runs! But still stalls. Argh.
Cleaned and inspected the intake snorkus - no cracks or tears. Tightened everything down. Checked every vacuum hose.
So I'm wondering if it isn't an AFM issue. If I open the manual AAV all the way, the car will pull a strong idle. If I touch it at all, the vacuum will drop below 20" and the engine will begin stumbling. If I close it, the engine stalls.
This is sort of weird, because if there's a vacuum leak, that means unmetered air is getting it, which should cause it to run lean. But the AVV is designed to let in unmetered air, and the engine runs better with it open.
I've also opened up the idle bypass, presumably letting in as much air as possible when the AAV is open.
I also rechecked compression.
The driver's side bank is lower, which I chalk up to the fact that a couple of years ago I had the passenger side head rebuilt, and the seals were pretty crummy. I imagine the other side is a little leaky with the factory seals still in place.
So maybe it's a bum AFM? Or a leak elsewhere? Maybe it's an injector seal?
How does the fuel pressure and voltage to the pump look when the engine stalls? How long will the engine run before stalling? Does the engine now restart after stalling, or do you still have to wait?
Also, let's clarify a few things: by "manual AAV," are you referring to the hand throttle pull-lever to the left of the steering wheel? If so, I think that "manual AAV" is a misnomer, as the hand throttle lever acts on the main throttle valve, same as the throttle pedal (in other words, it doesn't bypass the main throttle valve like the actual AAV does). I'm actually not really sure why the GTV6 has a hand throttle, as a properly functioning L-Jetronic system will cold start just fine without it (it is very handy for testing/diagnostic purposes, though!).
You are correct that the idle adjustment locknut mechanism (on the side of the throttle body in the early GTV6 like yours and mine, on the intake plenum itself in the later cars) acts upon a throttle bypass passage, but this is a controlled bypass that the engineers accounted for (within a nominal adjustment range), and the airflow meter is there to keep the air/fuel mixture in check (when all is functioning correctly, of course!).
Semantics aside, when the engine IS running, your vacuum reading does seem to be low, but the idle seems steady enough in the video. You are correct to check for vacuum leaks, but I just had another thought: is the oxygen sensor connected or disconnected? I would leave it disconnected to make sure a bad sensor or shorted wiring isn't affecting things . . . . .
Does the engine still stall if you barely touch the gas pedal? In your latest post, you say that the engine will stall unless the hand throttle is all the way open---my '81 GTV6 is missing the plastic insert between the hand throttle cable end and the lever in the engine compartment, which means there is extra slop in the linkage. Even still, I can raise the engine speed to over 3000 RPM with the handle pulled out all the way. Unless your hand throttle linkage is somehow even more sloppy than mine, something must be seriously out of whack!
A large enough vacuum leak to cause engine stalling will usually be fairly consistent, and it usually means that the engine will run OK-ish at higher RPM, but I am unsure if this latest symptom is the same old thing or something new (hence my initial question about fuel pressure). Based on your latest description, it seems that something is intermittently causing the air/fuel ratio to change massively.
If the fuel pressure and delivery volume situation is now OK, I would begin to focus on the airflow meter. Does the metering plate move freely without dragging? I am normally against parts-swapping, but it might be useful to swap in a known good AFM because it is so quick and easy . . . . .