I also have a 2001 Cadilac Seville SLS and Im wondering how do you access the cars "self scanning" system? I recenty had the "check Gas Cap" light come, I obviously checked the cap and its sucure, so I had it scanned and they said that Cylinder 8 was misfiring. They told me its probably the EGR valve, so I replaced that. I still have the same thing going on. Besides wondering how to access the scanning thing, do you have any idea on what else might be making my car act funny? Thanks much.
Ok, as for the gas cap light, even though you check it a it is secure, what is happening is that it is most likely failing. All gas cap are vented and there is a point at which it either quits venting or continues to vents. The gas tank it self requires a certain amount of vacuum when that vacuum is altered it will set the light. You can try removing it and tapping it a little then put it back on.
Ok, as for the Misfire in the #8 cylinder - there are many things that can cause this.
Now a questions for you.
1) When was last tune-up?
Here are a few other possibilities.
Coil Pack going bad
Crack Spark Plug Boot
Bad or Leaking Injector
And of course the EGR Valve - Put this last cause it would be the last thing I check.
About 85 - 90% of the time the plugs, coil or plug boot is the cause.