1998 Cadillac Deville white exaust smoke

I have the same problem only to less of an extent. I was told that the problem was a mixture of oil and carbon on my 97 seville (cadillac has a soloution to cleen it), But since it is asking for coolant, anti - freeze may be entering your block some how. It may be an intake maniflod or blown head gasket. The best way to tell if its a head gasket is if there is anti - freeze in the oil. You can also get a dye test done on it to tell if its a blown head gasket.

I just bought a 1999 DeVille in January. I believe mine may have sit a long time without being driven, and when I bought it, well just before I bought it, they changed the water pump and put a new Thermostat in it, and they failed to add the Pellets that Caddilac adds to the Northstar, plus they put the Green antifreeze in it, not the red that Caddilac demands. (GM Duramax, or other silicate free) it had a little white smoke, but went away after the car warmed up. Well on the advise of a friend who works for GM and used to build the larger Buicks, I added the Pellets. (Its actually those Bars Stop Leak Pellets, these cars come from the factory NEW with stop leak pellets!
Mine stopped smoking, stopped backfiring and stopped fouling plugs. I've put around 2000 miles on it since doing that. Sounds like yours is alot worse than mine though. I'd take it to the Cadillac dealer, I've found other mechanics dont even like to look at these cars. Good Luck! Jack