Ford F-150

After refueling a 1997 Ford F-150 4x4 with 4.8L V-8 the check engine light came on. The gas cap had been left off and driven for 7 miles. After replacing cap check engine light is still on. Any ideas would be appreciated. This has happened before and the light went out after replacing gas cap.
October 27, 2006.

I went to local Autozone and they read the trouble code which was P0303 which is a mis-fire on number 3 cylinder so they tell me. Since the spark plugs were replaced 4000 miles ago and the wires was replaced 2 years ago I'm assuming it's the coil but I will check the rest first.
When I got home with the pickup I stopped to get the and left it running. As I walked by the gas fill door I heard a soft whoosh, like a silent burb, sorry only way I can think to explain it. I've had this pickup for over 6 years and never heard it before. Now I'm wondering what is really going on.
Anybody got any ideas?

Oct 27, 2006.