I have a 2001 Expy with 225,000 miles and just had #4 coil replaced a month ago. Now #8 is miss firing. In the past I have replaced 1 and 6. Do I need to replace all the others while we are doing #8 or is it just one of those things?

 

Your choice. You can either replace them all preemptively as an attempt at preventative maintenance or one at a time as they fail.

You'll get advocacy of each course of action but it will ultimately be your decision.

 

Agreed ^^^ and the other thing will be which brand to use. Reading here I've seen too many instances of aftermarket COP's failing, my own experience with Granatelli says stick with Motor Craft or Standard Motor Products. Prices have come down to about $45 each from the various online parts sellers, probably find a similar price in your local outlets too.

Over three different E250's I've bought used (2 with 200K + miles) there's not be a single COP failure----I strongly recommend OEM all the way!

 

One thing to consider as well, is that more often than not, the COP itself has not failed, but the boot has rotten or torn, causing the spark to cross the tear into the exposed head instead of the plug, thus causing a misfire. Some auto parts stores carry the boot, which can be replaced alone (at about $5 apiece). Not always the case, but can be a much cheaper fix.

 

Good idea--NAPA carries a USA made Belden boot/spring package p/n #702418 for about $5 each!

Be sure to use a dab of dielectric grease in the plug and COP end, helps seal them.