Ford Repair: Check Engine Soon Light, spark plug wires, mercury sable

Question
Thank you very much for your quick response.  I appreciate it a lot.  
I'm confused however, would a faulty gas cap (you do mean where I fill my tank) cause a misfire to be read when a diagnostic is done? Especially a misfire in no 2 cylinder I did have a similar problem several years ago.  The light kept coming on no matter what I did, but it would also stall out as well.  A computer upgrade fixed the problem.  However, I did have a problem with the car doing the same thing from the same cylinder.  Sorry to seem weird - but this is extradinary annoying to me right now.


-------------------------
Followup To
Question -
I have a 96 Mercury Sable.  Two weeks ago my check engine light came on just after I filled the tank with gas from Arco.  I took it to my local mechanic and he said that the no 2 cylinder was misfiring.  He did a major tuneup, changed the spark plug wires, and cleaned the throttle body. He said that the problem probably was buildup inside the engine.  However, this morning, after filling the gas tank yesterday, I warmed up my car, got onto the freeway, and at the exact same spot last time, the check engine soon light again came back on.  The car isn't making any strange sounds that I can hear, and is driving normally.  Any ideas of what this could be caused by?
Answer -
Hello and thanks for the question...I'll try to help.
It would almost seem that the gas cap isn't getting properly seated and or is faulty. This has to do with the EVAP system in the car. What I would suggest is that you have the computer checked over, again, for fault codes and go from there. Also, have the gas cap checked and make sure when you fill up with gas that it is "clicked" in to place when you tighten it.
Hope this helps,
Erik

Answer
Hello and thanks for the reply...I'll try to help.
It wouldn't cause a misfire. But a faulty gas cap would cause the check engine light to come on. If the code is coming back for a mis-fire on the #2 cylinder, I would suggest that you have THAT particular ignition area checked out and also the injector for that cylinder checked out as well.
Erik