02 4.2L E250 missfire problem

This van has two re-occuring codes (Po301/po316)with a check engine light I've erased with my code reader several times. The no.#1 plug & wire have been replaced, and a fuel fliter.It lacks the normal power it use to have in acceleration.There is also a exhaust leak, just before the catylatic converter.A fuel cleaner additive was used that did help earlier.

When you changed the plug and wire, was the plug wet? Your injector could be clogged enough to cause a miss. I don't know how easy it is to get to the injectors on your engine, but you could switch that injector with another and see if the code follows it.

Dustin