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.
Tuesday, January 4th, 2011 AT 11:50 PM