The smoke unit stopped producing smoke. (or so I thought) . It ran fine for months then, one day, nothing. I disassembled the smoke unit per the instructions to check the wick. It was lightly charred but OK.
I decided to check the motor independently and it worked fine. I thought maybe the impeller was stuck so when I gently tugged it came right off and then rechecking the motor I noticed the motor made noise and I assume the shaft was spinning but the impeller was not. Compared to needing the strength of Hercules to pull out an impeller on a lionel smoke unit, this impeller slipped right off. it appears it no longer adheres to the shaft tight enough to spin, rather the shaft spins in the spindle freely. Never had that happen. I then reconnected the car wash to my layout, and hooked up only the smoke unit, no lights. Started it up and right on cue the smoke unit was generating smoke, the fan motor was running, ( I could hear it and feel the vibration) and smoke was very thick, but wasn't being forced out of the unit until I blew into the fill tube, then WOOSH! Nearly set off the smoke alarm, but when I stopped blowing, the smoke only wafted out... 
I believe this confirms the shaft is spinning in the impeller freely. Has anyone had this happen? Should I replace both the impeller and the fan motor, or just the impeller?
Thanks for your help.
