spark plug threads melted
 

I'm not sure quite how to discuss my problem, as I know little more about cars than how to drive mine. My car has been having problems, it's been running real loud, decreased power, and it kind of sounds like it is sucking air or something. I had my brother change the oil and spark plugs and he found that the threads of one plug were actually melted. I took my car to the dealership service department and without even looking at the car they told me that the head is stripped out and they will need to put a heli coil in for $400. I have seen the spark plug and it is definitely melted, my brother says for certain that the head is not stripped out, we just are not sure what could have caused the spark plug threads to melt.

A few months ago I had a problem with my check engine light coming on. I kept bringing my car to the dealership and they kept telling me to tighten my gas cap, like I was somekind of moron and too stupid to put it on right. Over a 5-month period I was in there at least 6 times, the first 3 or 4 they didn't even look at the car, then they ended up replacing the gas cap, the O2 censor and finally the gas tank. The engine light finally went off. My brother said that the code (something about a leak in the emissions control system) could have been causing the engine to run lean, which, over that 5 month period, could somehow have caused the spark plug to melt. Tomorrow I have to go down to the dealership and somehow prove to them that not only is the head not stripped out, but that their failure to fix my problem in the first place may be what caused the problem I have now. Does anyone have any information to support this or any other theory? Anything at all that could help? I would greatly appreciate it.