2002 Ford Expedition Possible blown head gasket

My moms 2002 ford expedition Eddie Bauer edition starting misfiring about 2 years ago, we were told that it was the result of the ignition coils not working because carbon was getting in them or something, so we hooked it up to a code reader and we told that the 3 and 6 ignition coils were bad, so we replaced them. It worked just right for about a month or so, but then the same thing started happening again, we hooked it up got the same code as the last time, saying that we needed to replace the ignition coils. This process repeated two more times. We finally took it to my grandfather who used to be a mechanic. He said that the only reason he could think of why it would be doing this is because of a blown or cracked head gasket. I let him look at it for a while and he told me that without getting into the actual engine block he wouldn't be able to tell. I decided not to let him do that because on the cars he's "fixed" in the past, these problem gets partially solved but something else is now wrong because he decided to "take a shortcut". I haven't been able to take it to a mechanic since because I can't afford it, how can I tell what exactly is going on myself without taking it to a mechanic, keep in mind that I work with computers, and don't have to much experience with cars, so put it in terms where a kindergartner could understand. I.E. The brake pads are the pads that touch the rim of the wheel base when you hit the brake pedal to slow you down. (I don't know if I even got that right.)