My Expy is making me crazy right now w/ random misfires that seem to be coil related, but its not setting codes. Once, at the very beginning of this, it had a little rough idle and when i checked codes it complained of #2 and #4 misfires. I reset the codes and went on with life. Then it got so it wouldn't idle worth a crap and my wife actually got so she didn't like driving it because she thought it would leave her stranded somewhere, but no more misfire codes, even though it had a way worse miss than when it did throw the codes. Finally, I changed #4 (since it was the hardest to get to, I figured that would be the one...) and it got much better, for a while. Then I changed #2 also, and its mostly okay but still has a random stretch where it will idle rough for a while and then be okay for quite a while. But, no more misfire codes, so I'm in the dark about which one to change next..... and at the price, I'm not going to just change them all....
The point of my long story is that the pcm is crappy at reporting which cylinder is misfiring, so don't assume that the coils are okay just because there are no misfire codes. And, if you spend some time searching other forums, you will find a number of guys who say that Ford pcm's don't always report the correct cylinder as the problem even when they do set a code....