Tiggeh

Gonna jump right into this, starting with what I've already done.

1990 FC, N/A

Fuel:
New fuel pump
Checked lines from pump to filter
new filter
new injectors
checked connectors for injectors
drained fuel and refilled tank with new fuel, no rust or anything

Air:
Checked everything in the intake up to the throttle body, no leaks
Replaced old corroded throttle body, set the tolerances on the butterfly valves
calibrated the TPS.
Checked MAF

Spark:
New spark plugs
checked wires
checked coil packs

Misc:
checked omp
replaced ecu
checked for clogged cat
checked all electrical grounds for corrosion, they're all clean
Checked for vac leaks

Known issues:
O2 sensor isnt connected, I just cannot for the life of me, I've done so much looking that I'm frankly convinced that one of the previous owners removed the wire for it or something.
clutch is starting to wear.

So, now for the symptoms. The car falls flat on its face under WOT (really from about 60% -100% throttle), and is impossible to get to higher than 5000 Rpm. When it hits that spot, it stops revving, rpms drop for a split second before the car stutters and backfires. If I need to supply a video I can to better see whats happening. If I go WOT at 3k rpm, the rpm will fall off a little, the car will spit and backfire, it'll rev back up to 3k, then repeat same goes for everthing up to 5k rpm.

The weird part is that whenever I'm below half throttle, and below ~4500 rpm, the car drives smooth as butter. no issues, no cutting, backfiring, anything, drives perfectly smooth and long as I take it easy.

Does anyone have any suggestions on what to do next? My next plans currently are to pull off the Upper and lower intake manifolds, check for any cracks/damaged gaskets, and after that replace the entire wiring harness. I'm just really at a complete loss on what to do next.

RotaryEvolution

iTrader: (30)

rear rotor housing grounds, even if they look good you may want to cut open the harness and see if there is corrosion or breaks in either off the ground loop to either of the harness wires.

DaBrkddy

I'd do what RotaryEvolution said, then try finding that O2 sensor connector and plug that thing in. It's usually dangling on the upper passenger side rear iron, just above where the intake manifold bolts to the block. It can only help...

I would also try testing your sensor readings at the ECU plug to rule out any wiring issues. I'd also replace the fuel filter. Also, what method did you use to check for vacuum leaks?

Edit: slap a fuel pressure gauge on it and drive it around to verify it has the correct pressure during these misfire conditions. It sounds like a fuel starvation problem.

Tiggeh

I have a new O2 sensor for it but I am completely unable to find the wiring for it. I believe the previous owner cut it out while doing one of his numerous "mods". Will try to rewire it tho.

I have two identical ecus for the car with the only difference being a slightly smoother idle. Fuel filter has already been replaced. For vacuum leaks we used the smoke machine at my buddies shop.

We did test fuel pressure while running but it was normal. I feel like I'm grasping at straws at this point.

misterstyx69

iTrader: (142)

That o2 wire is usually around the back side of the throttle body on the "passenger side" of the car,more or less.
If not then get the wiring pin out to the ECU and wire the O2 directly to the ECU Connector.That way you don't have to search endlessly through the harness..Just run the wire off the O2 to the wire at the ECU connector and splice into it.(you can cut it if you want in order to connect it but honestly I am not 100% sure if it goes to another part...so a splice is a safe thing to do.)
Satch should know which wire it is..( or go to Foxed.ca and look through the manuals)