Chrysler Repair: 88 Le Barron-emission test failure-smoking, poor combustion, emissions test

Question
I recently purchased a 1988 chrysler le barron.  When I took it in for an emissions test, it passed everything except it failed for smoke.  The testing facility was unable to explain why it passed every test except for smoke.  It also runs rough when idling in drive but smooths out when I put it in neutral.  Any connection between these two issues?  What do I need to do get it to pass the emissions test?
Thanks!

Answer
I need to know which engine you have and whether or not you have a turbo on the car as well.

White/blue smoke is typical of an oil leak in to the combustion chamber.  Older engines especially the 3.0L engine are known for leaking valve stem seals.

Thick white smoke is typical of coolant leaking in to the combustion chamber or exhaust at the turbo.

Black smoke is that of unburned fuel which obviously from poor combustion but a number of things could cause this ranging from sensors, vacuum lines or poor choice of plugs and wires.

I'll need more information about the car before I can help you better.
Doug