exhaust smoke??

hey guys i got a '91 caprice classic. it is blowing clouds of white smoke out of the exahaust pipe yet it doesn't do it all of the time. i could drive it for a couple of days and its perfectly fine and then i can get into it another day and it will smoke to the point where i can't drive it. Even when it is smoking it runs smooth. anyone know what this is?? thanks

Under what driving conditions does it blow the smoke? White smoke indicates coolant entering the combustion chamber either by head gasket, crack in the head, block, or entering in by the intake manifold. Take the spark plugs off and see if there is any sign of coolant on them. Probably do a compression check... or my favorite is a cylinder leakage test.

This is assuming that it isn't normal vaporizing condensation in the exhaust.