I could be wrong, but if you had way too much fuel mixed in (due to your o2 sensor) then grayish smoke could be released. Atleast that is sorta what happened w/ my friends nova. Carb was all fucked up and somethin was makin too much fuel be delivered, Caused grayish (maybe your white) smoke... that's just a guess though. It isnt that hard to change a head gasket to play it on the safe side..