Alright, I have owned a 1989 Toyota Pickup SR5 V6 auto for the past month or so. I bought it off a man who kept up with it very well. 140,000 miles, clean engine bay, everything was clean. I got the truck as a daily driver and have been doing just that. It runs perfectly. I have done a few routine things to it:
1. Replacing disks, calipers, and such in the front.
2. Replacing ATF fluid for steering.
3. Replacing brake fluid.

Basic stuff. BUT RECENTLY... I decided to take it in my back yard and replace the O2 sensor (upstream). I did that... super easy. Then i decided to find out where some of the buttons on the inside led to in the engine bay. Well there were a few stock buttons that looked like they were installed inside for some reason. So i basically just follow the wires and found out they led to the battery and a plug in... Just that one switch, because the others didn't have anything hooked up to them. So this one switch has nothing to do with anything... So anyway i tried to find out if any fuses were blown because my dash light dimmer switch wasn't working, i pull out and examine each one... Nothing. It ends up being the actual electronic dimmer knob itself... (It burned up). Anyways, i crawl out from under the steering wheel, put in the keys, turn to on and wait for the buzzing to stop (i assume its just a precautionary electronic buzz for all the electronics to catch up before cranking) all the lights work, battery is fine. So i turn the key to crank and it just turns and tries n tries but no actual start of the engine. Its like i just cranked the damn thing 5 min. ago. I try n try and realize that i may have run empty? But i just recently cranked it up n it was runnin fine... Anyways i assume i might have burned out the fuel pump but when i try n crank i can here the pump trying to send fuel... (After i had gone and got gas to fill up the tank with 5 gallons) all wires are good, all fuses are good, nothing seems out of place. I tried jumping the terminals on the diagnosis box to check for codes but they are all normal codes... Like correct operation and coolant readings and such. But nothing seems out of place. I also tried using starter fluid towards the throttle body and it almost started... You could hear it trying a bit easier but it still never cranked. I nothing seems to be working. Im worried i might have accidentally pulled a wire and never put it back but i don't remember doing anything like that. I live in MS and its in the shade so there are no weather conditions that should be affecting this. Idk what else to do. I also don't have my voltage meter with me... woo hoo! Any suggestions? Help please... I love this thing and need it soon.

 

So you went poking around disconnecting things now it will not start.

Either you have really bad luck and something failed random .

Something is disconnected or not plugged in correct

If it failed to start with starting fluid sounds like an ignition problem

In your case back to the basics Fuel , Air Spark, and timing it is the only way because even you have no idea just what you may have done.

 

I never disconnected anything. Only following wires from inside to outside is all. I might have wiggled loose something by touching wires and running my hands over them but i never disconnected anything. It had been running really rich before i changed the O2 sensor (thats why i changed it) so i assumed that maybe the spark plugs had lots of build up. Well whoever put the spark plug in there last didn't bother to put anti-seize on them so the end of the thread is stripped.