Okay, I have done a lot of reading through various forums and I cannot find anything alike enough to my problem to correctly diagnose it. I have both the fsm and the efi bible and have done many of the tests as I will explain.
To start off I have a 77 280z, f54 block, p90a head, totally stock. New ngk plugs and wires, gapped correctly, getting good strong spark. The fuseable links have been upgraded to maxifuses, the fuel pump is wired up to a kill switch. I have a fuel pressure tester T'ed between the fuel filter and the fuel rail to check pressure.
It ran strong and perfect for a few weeks after I bought it early last year and then I ran into some charging issues that were solved by cleaning grounds and connectors. But it sometimes would bog down while I was driving and chug until it died. I got it running for a few days by doing more cleaning, cleaning the afm and tips, making sure they were properly adjusted.
I wasn't able to get it started for a few months. I would turn the key and it would do nothing, I tracked it down to a bad battery and corroded starter solinoid, so I got new ones.
Now the problem was a strong crank but no start. I sprayed starting fluid into the throttle body and it would fire up for a few seconds and die, so obviously it was a fuel problem. So I started tracking more down. The dash was out at that point because I was doing repairs on it, and so I threw it back in, and as I was reconnecting connectors I found one that was on the passenger side that was disconnected yet didn't attach to the dash, I connected it, and without looking at my wiring diagram I'm pretty sure that it is one of the efi connectors, one that should probably have been plugged in.
Now that we have a backstory, about a week ago after installing the dash and reconnecting that connector me and my buddy Matt pushed her out of my garage and turned the ignition to on and turned on the fuel pump, the pressure gauge shows a strong 35ish psi as it should be, turned to start, and it cranked and fired right up!!
For about 2 seconds and then the rpm dropped down to nothing and it was dead. We tried again, this time with the throttle slightly depressed, it fired right up again, but this time, again the throttle about halfway depressed, it seemed to have an idle. However as soon as you let off the gas it dropped down in rpm all the way until it died. For the first couple startups if you revved it up it would respond and rev, however all the sudden, it would start with throttle depressed, but when you pumped the gas, nothing would happen, the rpm wouldn't move, as if you weren't doing anything, and as soon as you let off the gas it would die. Also, it was pouring out lots and lots of blackish smoke, which at first I assumed was just from accidentally over gassing the engine by turning on the fuel pump and cranking with no start. But it continued to put out lots of blackish smoke every time it ran. It also would backfire every once in a while, and a neighbor said it almost sounded as if it were only running off a few cylinders, however I've had no experience with that so I cannot know whether or not is is until I find a compression tester and do that.
For those that have read through and have any idea, it is extremely helpful. If you need I can give you more numbers and maybe even a video if I can get to it soon. I have the money to fix whatever I need and yet I cannot figure out what is going on. I know there is a wealth of knowledge flowing around in here, and I am ready to gain some of it.