Try unplugging the front O2 sensor, and going for a drive. See if the problem disappears.

 

I unplugged 02 sensor. No change. Unplugged air intake temp sensor, no change . Checked vacuum at idle 17 inches of mercury. Replaced pcv, no change.

 

17 is ok, but, maybe just a bit low.....

What do the various temp sensors have to say? Are the values they are showing you reasonable?

Compression and/or leakdown test??

 

Plenum gasket.

 

I did replace my the plenum gasket once before, but I only replaced the gasket, so this is a possibility. I plan to pull the throttle body and look around when it’s not 98 degrees outside.

 

I pulled and cleaned the throttle body. I didn’t think the plenum looked that bad. Remnants of bad pcv maybe.

Afterwards it ran great, until it warmed up, and then the same no power, backfiring mess.

 

Yeah, plenum doesn't look too bad. I agree on the PCV theory.

Something is getting hot, and failing. Have a scanner that can read/log live data?

 

I do have one.

 

I am curious what others oxygen sensor values are showing.

 

O2 sensors look good. Voltage should be rapidly switching hi/lo. (between 0 and 1 volt)