Hello everyone. I have a 2001 F350 6.8l and about 4 weeks ago I got a check engine light with code P0309 for a misfire in cylinder 9. I changed the spark plug and everything went back to normal. Then 2 weeks ago I was driving home from work when that same cylinder blew the spark plug. I had to use a helicoil to replace the threads and I also put in a new coil pack and spark plug in just that cylinder. Then Saturday I noticed I was idling very low and very roughly so I used the scanner and came up with the codes P0304 (misfire in cylinder 4), P0308 (misfire in cylinder 8), P0309 (misfire in cylinder 9), and P0359 (Ignition coil i primary/secondary circuit for cylinder 9). I dropped the truck off at my mechanic and he replaced the coil packs and spark plugs in all three of those cylinders and the problem was resolved yesterday. Now when I started my truck this afternoon I noticed the rough vibrations and found that cylinder 9 is misfiring again. Does anyone know what might be causing this cylinder to continue to misfire? I heard that the connector going to the coil may be loose but I wanted to see if anyone has had the same problem or has any solutions. Thank you

 

First thing to do is get that helicoil out and use a thread repair insert that will last.
Checkout info at - ++ TIME-SERT Official Threaded inserts for stripped threads, blown out sparkplugs,
They carry an insert that will repair a helicoil attempt.
How many miles on your plug boots? They can look fine, but be degraded allowing misfires.

 

Good points above. The fuel injector connector likes to come loose too - make sure that's tight.