Carbon fouled O2 Sensor:'92 VX - Perplexed

[soletek]

Possible cause: Pinhole leak in one of the exhaust passages in cylinderhead, remove exhaust manifold and see if one port has been super cleaned as if it was steam cleaned which is what a pinhole leak does. Low pressure zone created in exhaust manifold at port syphons coolant out of the high pressure 13-16 psi in the water jacket. The only fix for this is to weld up the hole or replace the head. I would suspect that the previous owner had this problem, ergo the replaced head gasket. Good luck.