Hey all,
I have a 3.4 swapped 2nd gen. Been chasing this issue for the better part of 5 days now. Lots of variables as everything on my car has been screwed for a long time but not like this.
What's happening is On a cold start, I idle fine, and get to 2k fine. After 2k it begins to groan bad and has absolutely no power. This is until I slam the gas off and on for a few minutes and have it drop gears (auto trans). Though, atleast regarding coolant temp, it is not solved once reaching OT. After about 5 minutes of doing this the car is fine. This stops it from lugging too much after (like 85% better) for the rest of the drive. Previously last week it was not doing that though, just wouldnt move without being at 5k.

I've redone the wiring on the MAF as it was beat, and cleaned it. Numbers appear within spec. 3.5-4.5 g/s at idle, ~40 at 2500rpm.

Fuel trims appear normal too less than +/- 10 on LTFT B1 and STFT B1. Except my B1S2 STFT sensor is permanently stuck at a reading of 99.2. But to my understanding, since its downstream, it does not change fuel trims and the overall bank reads the same as sensor 1. Instead just throws a cat code if it fits parameters.

Been running Technron through it, which honestly might be the reason it's cleared up, but just seems weird that it's mostly fine after a few mins of beating it. Doesnt seem conducive to a fuel pump/filter.
Wondering if its my trans (A340H)

Anyways long post, but desperate

Video is pedal to the metal, and also exhaust leak sounds dramatic but is downstream past sensors.
https://www.youtube.com/watch?v=LA2N7lGxvBs

Check the valve cover bolts
PCV
throttle body elbow
Coil packs
Spark plugs
Wires
Coil pack boot might have white ash marks

I have no good explanation of why but the downstream sensor does have an affect on how the engine runs. I had a mystery issue causing lean codes and stalling when hot that I chased for nearly a year. I replaced basically everything ignition and fuel related until finally doing the o2 sensor and it fixed it. Some people speculated than the downstream sensor only affects the tune long term which kinda makes sense to me but idk for sure

The downstream sensor is probably used to 'tune' the response of the main AF sensor on trucks so-equipped ('99/'00 CA emissions and all '01/'02's). Both work together...

-Charlie

I experienced a lack of power until I got open loop which is pretty much beating on it. Replaced the front o2 sensor and it fixed it. Accelerating in closed loop was a real slob.