97 Tahoe problems

Hi all,
I have a 1997 Chevy Tahoe LT 4x4 with 5.7 Vortec. The tahoe ran great for the first 300k miles, but then some issues started. At 321k miles the piston rings were shot and several other internal components, so I put in a used engine. At first it ran good, but 250 miles later it died again. Sometimes it has a no crank/no start condition, and other times it has a crank/ no start condition. Sometimes it will even crank and start fine, but run rough and sometimes when it starts it will backfire other times it won't.
I have replaced the distributor, wires, plugs, crank sensor, ignition control module, ignition coil, fuel pump, and fuel filter. Most of the parts had a warranty, so I figured it couldn't hurt to just replace them. I have checked the timing and it is correct. The fuel pressure reads good sometimes and sometimes not. I followed scanner danner's video to check the fuel pump relay and everything is testing good. The only thing that I noticed is there is no bias voltage on the switch power terminal. While the engine is cranking the volts read approx. 12.4 - 12.6, but when I stop cranking the volts drop to 0. If there is not bias voltage present, could this indicate a problem in the pcm?
I have checked everything that I can think of at this point, and everything is testing good except for the bias voltage. There aren't even any DTC's showing up. My scan tool is able to read data such as the coolant temperature, and air intake temperature, and throttle position, so I am thinking that should mean the pcm is good, but is it possible for it to be having an intermittent issue within the pcm?
Another thing that I should add is that the previous owner had spliced into a lot of wires and that has meant a lot of work for me to fix his mess ups. He even spliced into the 12v ignition signal that goes into the pcm directly next to the connector. I have soldered this wire since it had broke apparently from his splice, but I did that well before the Tahoe stopped running.

Thanks in advance for any help.

~Steve

how do you know the timing is correct? how did you set it? on those you need a scan tool that can read a piece of information called " cam retard" it only updates above 1000 rpm. that should read 0 degrees or very close to it. I don't know if a cheap obd scanner will read that or not. that will tess you if the distributor is truly in correctly. that's where id start. its easy and cheap if you have the scanner for the no crank when it occurs check and see if you have power on the smaller wire to the starter. you might be dealing with a failing ignition switch

I have set the timing on it multiple times since I have owned it and have just gotten used to it. I set the number 1 cylinder to TDC and turn the rotor to point towards the #1 plug. I don't have a reader for the cam retard, but have always had the timing set right with some slight adjustment ( it has always started on the first try, just have to turn it some to smooth it out). The problem had started while I was driving and it ran great up until that point. I checked the timing chain to ensure that it wasn't worn and that the teeth on the gears were in good condition.
I checked the starter wires and they are testing good. The power wire is supplying 12.6 volts to the solenoid, and the switch wire is receiving power when the key is turned to the Run position. I had the starter tested and it is testing good as well.
I did wonder about the ignition switch, but the only tests I could find for it were to check the presence of power at the crank fuse when attempting to crank the engine, and there is always power present when attempting to crank the engine. Also when attempting to crank the engine, there is voltage in the fuse box for the fuel pump relay power switch side. If you know of any other tests that I can perform to further diagnose the ignition switch, I would greatly appreciate the information.
Another thing that I noticed is that the CEL illuminates when the key is turned to the ON position most of the times. Occasionally, the CEL does not illuminate, but after a few more turns of the key off and on, it will illuminate again. Any ideas on this?

~Steve

keep an eye on the check engine light NOT illuminating. when it doesn't illuminate check and see if its a crank no start. if it is a crank no start when the check engine light is not illuminating check for 5 volts at the tps sensor. if you have it then id focus on powers and grounds at the pcm

I did happen one time when I hooked up my scanner that the CEL did not illuminate, but it did show a reading for the tps and I pressed the accelerator to see if the value would change and it did, but I never checked the voltage at the tps itself when the CEL didn't illuminate. I will check it next time I notice that it doesn't come on.
What is the best way to check the powers and grounds at the pcm? I have checked some of the connector pins for power, i.e. the 12v ignition feed, and they have tested good. There are 5 connectors total though and I have only checked the pins on one of the connectors since one of the wires needed repair due to splicing from the previous owner.

Thank you for helping me walk through this process Cheryl, it has been a headache and I have reached the end of my knowledge with the diagnostics.

~Steve

Hi Steve, welcome to the community!

Sometimes it has a no crank/no start condition, and other times it has a crank/ no start condition. Sometimes it will even crank and start fine, but run rough and sometimes when it starts it will backfire other times it won't.

The intermittent no crank and erratic fuel pressure are making me think of a poor block ground or battery connection. Perhaps the result of the engine swap. Not set in stone of course, just a thought.
It could be two separate issues.
Here's a video on troubleshooting the fuel pump electrical circuit that should be very similar to your Tahoe.

And another one featuring a no crank scenario caused by a poor block ground.

And one showing how to check an ignition switch.
This one is a Jeep Wrangler, so the wires will be different. But as long as you can get your hands on a diagram for your truck, the testing will be very similar.

I tested grounds today and noticed 0.46 v from the battery negative to the engine block while cranking. I hooked up a jumper cable like scanner danner did in the one video that you posted, but it made no difference. Also the wire that runs from the pcm to the icm is only measuring up and down from 0.16 to 0.2 v and while cranking and doesn't pulse with a test tight. Another thing that I found odd was that the switch power terminal for the fuel pump relay only works occasionally, could that be a ground issue or a pcm issue?
The maf connector is reading 8.6 v on the yellow wire which I believe should only be 5v.