they are wrong and worn out. when the center looks like a well worn pencil, it's time.

the correct plugs are fr8dpp33 and have a much finer electrode. usually about 10 ea usd. which is why the wrong plugs are there-cheaper. the real plugs will go 60k or more. i don't like to go to 100k like mb says to do because the coils and wires get tired from the extra amperage required for older, worn plugs. good luck, chuck.