Carbon fouled O2 Sensor:'92 VX - Perplexed

[Darkelf]

After initial success in trying to eliminate my "miss under load/ bucking under light load" issue, the situation has gone south rather badly.

My O2 sensor (new, by previous owner-not more than a few months old) has caused a CEL for the LAF (code 48) and is now stuck in open-loop.

There is heavy carbon sooting on the sensor tip and I strongly suspect it being from coolant as there is a mysterious loss of it from the radiator. NO leaks can be found. Anywhere.

The perplexing thing is tho, that there is absolutely no fouling of the spark plugs. I would expect there to be some if the engine is burning coolant/oil, yes?

How could contaminates bypass the combustion chamber? I have to assume this contaminate is being burned by the hot exhaust gasses and then depositing carbon on the sensor.

I noticed carbon on the sensor before I did an IM rebuild but just cleaned it off.

Things that are and are not:
1) Can't see or smell anything out of the ordinary from the tailpipe. When first started or when warm.
2) There is no evidence of oil/coolant cross contamination.
3) No overflow in the coolant reservoir.
4) Suspect head gasket to be recent -area around it very clean and it just 'looks new'.
5) Have recently rebuilt the IM and systems [See the thread :"Air Intake System rebuild questions" for more details of what was done]
6) I rechecked everything I did in #5 and can't imagine what I could have screwed up but am open to suggestions.
7) Don't suspect oil leak - oil level stable
8) Suspect Coolant leak but WHERE???

{whimper} help?