S2K

A buddy of mine has a 2002 F-150 2wd 4.6 sohc. It threw a code for misfire on cylinder 6. The dealer told him it was the coil. Well we replaced the coil for #6; next day same code popped up. Anyone have any ideas as for why it did this? I plan on switching 6 and 7 coils to see if it moves or not in case we got another bad coil or to see if it wasn't the coil at all. Also, I changed all the plugs. Number 6 didnt look bad on the combustion side, but was red on coil side. There were also 3 plugs on the driver's side that the theads were coated in oil and looked like ****. I'm pretty sure they were the stock plugs on an 8x,xxx mile truck.

Any help is appreciated. Thanks. - Todd