Another possible cause of white smoke out the exhaust is a ruptured diaphram in the vacumm modulator of an automatic transmission vehicle. This white smoke will be a drier loftier white smoke as opposed to a heavier steamy white smoke caused by coolant. It is the result of automatic transmission fluid being sucked into the intake manifold by engine vacumm and burning in the combustion chamber(s). It will be most obvious under high vacumm conditions ie coasting with the throttle plate(s)closed.

I’m dating myself by providing this information as most late model vehicles transmissions are electronically controled and vacumm modulators are not seen much on todays cars.