Well, this is my first post, although i've been using this GREAT N WONDERFUL site for 1.5 yrs. Since many senior tech'ies are always around to answer, I never got the courage to post reply ( fearing i may sound like a dumb fool).

On this particular issue of White Smoke in 1993 S600 (or 600 SEL) V-12 Chassis W140057, Engine 120980, i think i can shed some light since i went through the same pain/ordeal as HerbPhillips. Only difference being that my intake is contaminated from black sooty oil (rather than clean crankcase oil as Herb suggested), probably coming from PCV inlets at bottom of EA/CC/ISC throttle actuators.

Herb - DON'T THROW PARTS AT IT YET !!. You need to do a few things as listed below :

1. Check your Air Mass Sensor (MAF) wiring harness. I'm sure it has lost insulation. Replace MAF wiring harness upto the computer box (very important).
2. If wiring has lost insulation, u may need to replace the MAF sensors also, as these go bad due to shorts in wiring. Make sure that rubber hoses that connect MAF to intake manifold thru EA/CC/ISC actuators are not cracked.
3. Check that two purge control valves of EVAP system are functioning properly - heartbeat effect while engine is hot. Check the charcoal canister line for clogging (carbon deposits).
above were major source of white smoke for me.

4. Check all Vacuum lines for cracks, clogging/blocks. Clean/replace as neccessary (Very important). Vacuum lines towards the windshield are difficult to check.
5. Check that all vacuum/suction points of Intake manifold are open (not clogged at the intake manifold itself).
6. Clean, as far as possible, black pipes/lines that run over the engine for Positive Crankcase Ventilation system.
7. Clean distributor cap, and other ignition contacts (i presume ignition componets are working fine) - check that all 12 spark plugs are firing.
8. Check key sensors - Two engine coolant temperature sensors (ECT), Two Intake Air Temperature sensors - for proper resistance rating.
9. Replace air filters, engine oil/filter, tranny oil/filter etc.

Have patience, there are 'n' number of things still to check - CATs, O2 sensors etc. Don't jump to conclusions immediately after doing all of the above. Run the car in various driving conditions for 300-500 miles and see your white smoke problem go away.

If the car was sitting in the Garage for some time, I would also recommend to take out all the modules (ICU's) from computer box and clean all/EARTHing contacts (module connection brackets, side contacts on module box etc.). Clean other electrical contacts as much as you can.

This car has attitude, if you respond with care, it will correct itself.

Keep us posted. This problem can be solved and will be solved.