Bad Valve Seal????

I have a 3.8 99 Mustang. O.K actually my wife does. Anyway the cylinder closest to the driver keeps collecting carbon on that plug only and now it will ping like crazy when getting on it. I suspect the valve seal is bad and I have a manual. My question is is there anything else that could be causeing this. The book makes it look like an afternoon project but is there anything that I should know about before I tear into this. One more thing this thing has sounded like it has had an exhaust leak since day one but the ford house could never find anything and stated that they didn't hear any abnormal noises. Anyway any help would be great.

Thanks