Another one (I learned the hard way) is to put the wrong anti-seize compound on the thread thinking it would help by the next removal.

The correct "grease" is a compound that contains glass particles mixed with a sensor safe fluid. Once you start to unbolt the sensor the glass particles brake and free the thread.
If you use a copper anti-seize compound the grease and the copper may come in contact with the sensing part of the sensor changing its properties = slow response + wrong readings.
That's the reason why I replaced my O2's last time...

New Delco O2s come with anti-seize on the threads so no need to add any.

A short update: I'm filling now a fuel with an higer octane grade and the fuel consuption dropped significantly.
I had no KR with the old fuel, maybe it was just chemically different (with some ethanol?)

I suspect more ethanol being used in some grades or who makes the gas.
As I mentioned we are seeing anywhere from 10 to 15% less mileage depending on gas station we tried

Update:
The fuel consumption is now at 24 mi/ga on the highway.
This driving at about 75 mph during 200 miles.
Starting at 890 ft, climbing the Alps up to 3800 ft and than back down to 1800 ft.
I think it's not too bad for a '98 convertible F-Body with 85'000 miles on the engine.

Somehow the fuel consumption went back to what it used to be...
- Maybe a different fuel
- Maybe the oxygen sensors and the cats ane now clean

(I started this thread after an engine flush + top engine cleaning what maybe transferred the deposits from the combustion chamber to the exhaust).

The only difference I see are the fuel trims: while consuming 20 - 30% more fuel the trims were about -2%, now they are at +1% in average. I don't know if it means something.
The exhaust sounds louder too

Maybe I shouldn't care too much about fuel!
High grade fuel is now at 6.97 $/ga, diesel at 7.88 $/ga


Greetings from Switzerland

Stefano

Wow $7 a gallon that is a real rip off.
Gas here is around $4 and it sucks to spend $80 for a tank of gas !

I suspect the problem was the O2 sensors, dirty injectors and carbon buildup and 24 MPG at 75 MPH is pretty good with that many miles on the car and driving through the mountain hills.

Glad you have the problem solved and your LTFTs are right on.