Always start a question with year, model, engine, transmission.
These things changed so much over the years that we really need to know that.
Based on your user name Im going to assume that this is an 89.
Theorectically 95 and older are all OBD1 systems.
OBD11 is 96 and newer.
But, 95 vehicles are so odd that they are pretty much a "stand alone" one year only variation.
You can check for codes using the paper clip trick on an OBD1.
Really easy to do.
As long as you have not disconnected the battery or let it sit untill the battery went dead.
If you disconnect the battery on a ODD1 system all of the parameters that the ecm uses to control the engine are lost.
And so are the codes.
When you restart it again it needs to "relearn" from those inputs before it will run properly.
If this is an 89, it has two temp sensors.
One in the passenger side cylinder head.
That one feeds signal to the dash temp gauge
The other one is on the pass forward side of the intake manifold.
That feeds the ecm.
Whenever you crack a sealed system open you have to purge the air out of it.
If you pulled the head sensor and let it drain I gaurentee that you air in the cooling system.
If you pulled the manifold sensor and it was not full right to the top?
You have air in the system.
At hot idle I would be kinda surprised if it caused the ecm to firehose fuel like it is but it could be some ecm base module to keep it from being to lean.
Even over my crappy celll phone it looks like a major injector pulse issue.
Why did you replace the temp sensor?
What was going on before that?
Were you having problems before you replaced it or was everything fine untill after you replaced it?
Give us the back story with as much detail as you can.
I think you have maybe an air problem but you need to check fuel pressure for sure and set your timing with the dist wire disconnected.
Start with the basics.
Also a
good thing to note is that in GM cars (gm trucks I do not believe did this) many had a Obd 2 shaped connector by 94, trucks by 95, and most (but few exceptions exist) were obd2 compliant by 96 if not at least partially (as indicated on the VECI usually on the hood sometimes on the core support areas) The obd1 with obd1 connector (ALDL in gm literature pre 92 ish, but called the DLC otherwise) is rectangular , and the obd2 shaped obd1 connector has the same functionality but may be missing field service mode depending on the car, and the pinout is slightly different.
But then there is obd "1.5" which doesnt have the same function as obd1 but it doesnt technically work with a lot of generic obd2 tools, but few have found ones that have support for them before. My tech 2 , for example, works pretty darn good on the 1.5 vehicles as I tested out on a 1995 vandura van, had about everything a 1996 would have that I could tell(functions), had the same syntax for codes and definitions. etc)
(did spoiler because no relevant for the OP, so no need to read it if you werent wanting to talk nerd stuff)
Reminds me when helping someone on buick fourms found a 95 park avenue had a 1994 C3500 detroit diesel PROM in the ECM(ecm was bought buy prev owner) the thing ran like ass, and it was a good insight into how the ECM can figure out how to run an engine with very basic hardware calculations without any specific software. couldnt run the transmission well tho. lol.
on that note,
grounds
The early (if not all) gmt 400 use an ECM ground on, at least small block v8 design, the thermostat housing. I didnt think about this until I saw you mention coolant temp and bleeding. The ground needs to be clean and the engine to battery ground needs to be good. Ideally, the ground for the pcm should not be , in my opinion, on the engine , and moved to the fender where it would not be in a ground path for any large electronics, and no other device should ground on the same spot. The ground on gmt 400 is either stacked on other grounds or was next to the large engine to battery ground cable IIRC