Before I spend more money into the van basicly just want diag steps to pin point exactly what is wrong.

The Van started lack of power at above 3k rpm and misfires can be heard and felt. checked for codes found p0305/p0306.....went to the dealer, dealer says veh needs coils for 5 and 6. Also advised a tune up since it has 109k. Since I know my way around a engine I decline the repair, buy the parts, and get to work.
replaced:
coil packs for 5/6
all 8 spark plugs
pcv valve
fuel filter

started the van, noticed the van would not rev pasted 3k rpm in park. test drove, first time above 3k rpm under load misfire was felt flashing check engine on then stay on. checked codes p0305 came up. figured I would swap injectors from 5 to 6. cleared codes. test drove the van again under load above 3krpm and it started misfiring, p0306 came up. Thought the problem was figured out bad injector. Installed 1 oem injector for 6. test drove veh, same thing under load 3k rpm misfire. scanned veh p0305. now completely confused, moved the new injector from 6 to 5. put the veh back together and drove it. again underload at above 3k rpm misfire again p0305. so number 5 has new injector,plug,coil.
Read in another forum about cats melting blocking exhaust flow. What is the life span of the cats on the truck? I know my cars manual say the cats only lasts 100k miles, butr the car has 175k without a problem.Is there a way to test this? Is there anything I am overlooking for me to check? I am at my wits end, any opinion or input would be great. Thanks in advance.

 

Thanks for all the response, they were all helpful.
so it was easy to test, I removed the precat o2s and the truck run like it was supposed to.
got a new y pipe, installed it.....truck runs good as new

 

Glad to hear it. And thanks for the report. Welcome to FTE as well.