Hey guys,

   Ive recently been driving my 87 V6 GT since the weather has been so nice
and Ive developed a problem that Im not sure how to address, or even what it
is, but Im pretty sure they are all connected:

   First issue, when I drive 80 on the highway for about 4-5 solid minutes
without much variation in speed, my check engine light comes on.  Once I
park and do the old trick to check the engine code, I just get 12 over and
over, no problem found.

   Secondly, Ive been getting a large amount of soot on the back of my car.
Its not greasy or anything like that, its just some black spots with a
coating of black powder on the whole back of the car.  If I tug on the
throttle engine cable, the exhaust blows smoke, but it seems much more white
than blue, and it reeks of gasoline.  I just got the engine rebuilt about
800 miles ago, and Im hoping that I didnt screw up the piston rings or
valves yet.  Ive been thinking/hoping that the engine is running rich and
not burning all the gas, as opposed to burning oil.

   Lastly, which might not be a connected problem.  If I turn on the car
without pressing the gas pedal, it will stall out.  If I turn it on and just
pump the gas once its ok.

   So, I have talked to a few people who said my timing might be off?  It
does indeed run a LITTLE rough at idle, and I mean a little.  Nothing that I
would think suggests a timing problem.  Im hoping maybe somekind of sensor
is bad, but I dont know why my ECU doesnt record it and report it when I do
the check engine thing.  If anyone can tell me what even to TRY to replace
or check out, Id be willing to give it a shot.  I was hoping to get it
painted this month, but if Im going to get soot all over a brand new
paintjob every 3 days, to hell with that idea.  This is really starting to
irritate me, and if the piston rings have gone bad there will soon be a
Fiero for sale on the newsgroup, as Im about finished with trying to get the
car in great working order.  =)  Any suggestions are extremely helpful.
Thanks in advance you guys!

Nate