loose spark plug wire boot
 

I had been getting some smoke and slight vibration, so I checked my wires. (They were replaced with OEM about 5 months ago).

Lo and behold, the 3rd wire from front on driver's side (420 SEL) was loose at boot. Upon removal, I found that in addition to being loose, white powder gunk had clogged the boot and the inner recepticle preventing it from properly inserting all the way.

After I cleaned it out and plugged it all the way in, car seems to run stronger with less vibration.
Also, smoke appears to have disappeared.

Does all this seem logical?

Perfectly logical. With a poor electrical path from the distributor to the spark plug, you weren't getting complete combustion in the #7 cylinder. The smoke you noted was unburned gasoline.

There can be other reasons, but if your car runs as you feel it should, give yourself a big pat on the back for a job well done.

Sixto
91 300SE
81 300SD