OK, I finally have time for my report...this is how things played out.

First, after all said and done, I was not seeing 12V (most of the time) at the rear connection when the ignition was turned on. This is absolutely abnormal under any circumstances, which was discussed. The pin-out diagram was very useful when I went back and double checked my readings; as you guys had suggested, I may have been misreading the diagram initially. When I went back again, there was no power at any voltage on the primary fuel pump circuit.

I then moved up to the under-hood fuse box to see if we had power problems there. I took apart the fuse box to get at the wires exiting the rear of the fuse box and look for the mentioned GRAY fuel pump primary power wire. There was no power there when the ignition was ON, so I thought I was getting somewhere. I really disassembled the box to get at the wire connections underneath and check continuity on all the wire, relay, and fuse connections. Everything looked and read good with no circuit breaks. I was starting to second guess if I had properly tested the relay (which I had). There was definitely 12V on the high amp side of the relay, but there was intermittent power on the switched low amp side. I decided to just jump and bypass the relay with a wire as an experiment to check the circuit and see if the pump would just run...test that circuit. It did indeed run, and run, and run...so I concluded the issue was in the low amp switched side of the circuit. What was a surprise to me was that was it wasn't cutting off when the pressure built in the fuel line. That's when I had the BIG realization of my misunderstanding of the circuit!!

I had, before that moment, a misunderstanding how the fuel pressure regulator cut the pump on and off. I had thought (incorrectly) that it functioned/switched at the pump itself on the high amp circuit. I had just realized that the fuel pressure regulator switched on the low amp side of the relay!! At that moment, I realized my problem could only likely be one of two primary things interrupting the low amp switched circuit. Either bad wiring or, more likely, a bad fuel pressure regulator switch in the new pump I had bought and installed a couple days ago.

Well, I did have a new fuel pump which had just arrived in the mail. I'm getting pretty good at this now and was able to toss in the new pump, fuel gauge sender, fuel pressure regulator switch assembly in an hour.

As soon as I tossed it in, the car fired right up!! Now, realize, this is my second new pump...guess what, there was another problem. The fuel gauge was showing zero!! WTF!! I thought maybe it was the plug or something silly like that. Well, after dropping the tank a third time and doing some quick trouble shooting, I quickly found that the fuel gauge sender on the 2nd NEW assembly was also faulty!!! I couldn't believe my bad luck. Two separate new fuel pumps had two separate faults. I pulled a good sender off one of the other two fuel pumps I had and soldered and installed it in place of the bad one.

This time I was sure to test everything before I reinstalled the tank. It all tested fine, so I closed up and installed the tank a 3rd and final time. I turned ON the ignition and there was a gauge signal...I turned the key and it fired to life right away! :)

I zeroed out the computer codes and have been driving for 24 hours plus with no issues.

OK, Lessons learned...

I wish I had a full wiring diagram early in the diagnosis process as it would have let me diagnose fully the bad new pump earlier. It's my fault for not seeking one out myself.

Second, I wish I had read the pin-out diagram at the rear of the car by the fuel pump more closely, but in the end, I believe we had intermittent 12V power at that location due to the intermittently functioning switch at the faulty fuel pressure switch in the fuel pump assembly.

Third, as noted early in the diagnosis thread, I wish we had invested in a "Brand-name" fuel pump with the hope the original installation would have been successful as well as the second new pump's fuel gauge sender functioning as expected.

Lastly, the good news is the ebay seller for the first pump gave me a full refund with out even sending back the faulty pump...weird, huh? That was also where I sourced the good fuel sender in the final install/repair.

Whew! Thanks guys. You cannot imagine how challenging this has been. A simple fuel pump install gone bad...

I hope someone else benefits from my mistakes and all the info in this thread.

 

Glad to hear you got it figured out and fixed.
Also. Great write up and explanation on how you tracked down the problem. We now know a few more details of how that particular power feed circuit works.

Thanks again for the follow up. I hope that new pump gives you many miles of trouble free use.

 

You might want to post the brand. I think I know what it is. Does it have "air" in it's name?

 

Glad it's all worked out, this is why I always recommend an OEM GM or AC Delco part!!
You just can't trust the others, as your experience has taught you, but hey, you're gonna be the first guy we call on for tips to drop the tank and what to look for!!