Okay, this has been puzzling me for a few weeks. I will go into detail over everything I've done so far. I'm not sure what the problem is. PLEASE READ ENTIRE POST

1994 180sx with a built SR20DET redtop swap. Top mount, big turbo, Apexi FC, aeromotive 340lph

When I first start the car from cold it starts up no problem and runs excellent. No noises, no issues. If I wait about a minute, OR I try to give it some gas the engine will bog and start to run like crap. It SEEMS like when it goes into closed loop it will run poorly?
If I try to apply consistent throttle it will rev up but it sounds like an ignition cut rev limiter almost.

I've checked the fuel pressure, tested the MAF, tested the CTS, tested & replaced the TPS, checked the plugs, checked the coils.

I found the TPS sensor to be bad and reading 5V all the time so I replaced that to a working unit. Sypmtoms have improved but not much.

I used the power FC to check sensors and it looks like everything is reading right, except I am not getting a reading on the O2S. I tried disabling o2 feedback from within the power FC and nothing changed.

I honestly have no idea what could be causing this right now especially cause when it first starts it runs perfectly and then starts to get messed up.

Any ideas? TIA!

Not enough there for the sensors to be working, one needs to be able to input the correct moves (fuel, spark advance, etc.) that create the software needed there, if that system is a true standalone one. You'll need O2 input and from wideband there. Why the Apexi site says to.

Here's an update: I went over the harness again. Buddy I bought the car from had the harness from one car and the engine/trans from another. The little brown wire near the fuse box on the RHD passenger side was not connected (therefore no power to the o2 sensor). I wired this in to a switched 12V+ source and the o2 sensor is working. It has, however, made the problem worse.

Now when I start the car it will run fine but only for 5 seconds or so and then it will die out completely. There is poor throttle response but if I'm lucky it will rev up and run okay if I continue to apply throttle. I've tried to disable O2 feedback from within the Power FC controller but nothing changes.