Did you check this site that I linked to earlier:
“I have a fuel-oil furnace that white smoke appears through the burner after the burner shuts off and the blower is still running, after about 45 seconds...white smoke starts to come out of these small holes on the burner and fill the basement with white smoke and smells like fuel.
I changed the air-filter about a month ago and the oil-filter, too. I had to install a new blower motor like 2 months ago because the bearings went up...and just recently I decided to open the inspection cover and look into the furnace as it was running and I could not stand the heat. It blew out and if I was within 4 feet of directly in front of that inspection door the heat was unbearable and so I tried to look in from an angle and noticed that the flame inside was about a cubic foot in size and was bright yellow. I don’t know if it supposed to be blue or if that is only on natural gas furnaces or not.
I looked up the chimney with a flashlight and mirror from a cutout in the bottom where it seems years ago there might have been a coal burning stove and I could see a square sunlight at the top, so I figured the chimney is fine.
The swinging small door, I think is a damper swings fine but seems a little weak. I really don’t see any smoke in the flue.
The furnace works fine... I am just concerned about the white smoke that appears coming out from small holes in the burner assembly itself. I did just 2 weeks ago vacuum out a lot of rusty scale buildup inside the furnace and in that inspection door I used my hand to sweep out a lot of rusty scale buildup, too and it seems it fell into and all around the burner assembly since it is positioned directly underneath where I was trying to clean, but I used compressed air after I seen smoke and blew out as much dirt and rusty scale buildup that I might have accidentally knocked into the burner.
I haven’t taken the burner off or anything like that but I might 1 day to clean the nozzle, when I read more on how to do it ...but I am just concerned about that white smoke and it starts when after the burner fires the blower comes on and then the burner shuts off and the blower is still on then after 45 seconds after the burner shuts off and the blower keeps going the white smoke starts to pour out around the burner and wont stop until the blower finally shuts off...Any ideas? Thanks!!! “
I think you are onto it; bad draft and the blower kicking it back out.. My many thanks for the link and subsequent. Going to tear some stuff out now looking as the home is warm!! :)