I'm using Delco plugs and have double and triple checked the firing order.  Haven't done the inductive timing light thing yet, so thanks for the idea.  Thing is, it doesn't feel like it's a cylinder not firing.  I'm wondering if an electrode could have gotten damaged when I regapped them and is only partially firing.

 Charlie wrote:
 > What do you gap your plugs at.  The Haynes manual says .050.  I called
 > the dealer and talked with a mechanic and he said to gap them at .040.  
 > When I pulled the plugs, the old plugs were gapped around .068.  From
 > what I assume, the old plug gap widens as they get worn.  Well I changed
 > the plugs and wires on the car and gapped them at .042 and changed the
 > wires.  The car now runs rough and the Service Engine Soon light comes
 > on when the car warms up to the point where the secondary fan comes on
 > now.  I've double checked all the wires and they are in the correct
 > order and are on tight.  All the vacuum hoses are connected.  I can't
 > see anything that I could have done to cause the problem.  Could the gap
 > being too close cause it to run rough?
 >

 You didn't say what plugs you are using. I would think .050 for Delco
 plugs, but I wouldn't think .042 would make a huge noticeable difference
 in how the engine runs. You need to recheck the plug wire installation
 carefully to see if you might have two wires cross-connected (wrong
 firing order/cylinder), and use an inductive timing light on each wire
 to make sure all cylinders are firing

A buddy's owner's manual for his '92 agrees with Haynes at .050" I think
the dealer mech is playing a "one size fits all" game --the L-98 gap was
speced at .035" and he's probably splitting the difference for all C4s.
The old rule of thumb was that a wider gap smoothed idle and a narrower
gap helped at high speed, high torque loads or with cold weather
starting.  With HEI systems and particularly with Opti-Spark, plug gap
is no longer a fussy thing--ignition timing is very accurate at all
combusion pressures. Over time HEI does vaporize the electrode material
and the gap widens.

Sounds like you've got incomplete combusion in one or more cylinders --
and the ECM is going bananas trying to correct the downstream problem
at the oxygen sensor, then the ECM throws a code.  By the way, what code
is it throwing?

If the code is no clue then look at the changes you made. Cracked
ceramic on a new plug?  Unseated plug that's blowing by? Loose or
punctured wire? Wayne's suggestion is good and should show the problem
unless you've got a partially shorted plug. If you lack an inductively
triggered light, find a "tune up" guy with an old fashioned ignition
analyzer. It will show the ignition pattern on all eight cylinders,
shorted plugs, bad cables, hangnails and frustrated sex drive. Pick the
sour one and work just on it.