Cylinder 8 Misfire - DTC P0308

[BlueStangGT]

This popped on me a couple of days ago. Motor started running and idling rough and eventually produced a flashing CEL. Stopped at Autozone to get the code pulled since I didn’t have my programmer with me. Both the CEL and rough motor cleared up after a couple of driving cycles. Had another brief episode yesterday but no code produced. COP maybe?

07 XLT 4WD 120k miles, original COP’s. Plugs changed at about 112k.

 

[Dennis_H]

I fought one of these for a bit not too long ago on my 2010. Mine was cylinder 3 though. Had changed plugs and boots, that's when it began with a little miss. Decided to change COPs too. Stayed the same. It ran OK after about 15 min, and only threw a code if I really got on it. Idle had a miss and you could feel it when running. Ended up buying another plug, putting on an old coil and it runs like a top. Think one of my new plugs just didn't work right. Never had one do that before.

My dad had exactly the same type symptom on his V10 engine. His turned out to be the fuel injector. That was my next try if the plug/COP change didn't work.

With yours, since it didn't do it close to when plugs were changed, and cyl 8 back by firewall, COP would be first choice.

 

[vintageman]

I would start with just the boot and spring first on it instead of buying the whole coil pack. I had a similar issue with a couple previous triton engines of mine and just changed out all of the boot and springs and the issues were corrected. You can get a pack of all 8 at autozone for probably around $30 or so. Use some dielectric grease and will probably be good to go. If that doesn't fix it, move that COP to another cylinder and if you get the same code later, change the spark plug. If the code moves to the new cylinder, then its the COP. Good luck.
Tom

 

[ExpeditionAndy]

If you have reused the COPs, did you change the connectors? The connectors can become galled causing high resistance at the plug (this can cause the COP to fail). They are easy to check and they should be clean and shiny, some electrical grease is good thought to help seal the boot and make good contact with the top of the plug.

 

[scobar]

Did you put any dielectric grease in the boot after the plug swap?
I had an intermittent misfire that was resolve with a new OEM COP.

 

[Adieu]

I would start with just the boot and spring first on it instead of buying the whole coil pack. I had a similar issue with a couple previous triton engines of mine and just changed out all of the boot and springs and the issues were corrected. You can get a pack of all 8 at autozone for probably around $30 or so. Use some dielectric grease and will probably be good to go. If that doesn't fix it, move that COP to another cylinder and if you get the same code later, change the spark plug. If the code moves to the new cylinder, then its the COP. Good luck.
Tom

You can also get all 8 COPs for $30 total (generic) for the old type spark plug versions, including 07 afaik.... newer type ones start from double that though

 

[BlueStangGT]

Thanks for the replies everybody. If it happens again I’ll start the process and post any relevant findings.

 

[BlueStangGT]

Well, it happened again yesterday. It was the COP, got it replaced and everything seems to be working well once again. Thanks again for the replies and suggestions.

 

[Adieu]

Well, it happened again yesterday. It was the COP, got it replaced and everything seems to be working well once again. Thanks again for the replies and suggestions.

Did you go with a single Motorcraft, splurge on 8 Motorcrafts, or buy a set of budget generics?

 

[Trainmaster]

Remember which coil it was, because if you bought a $20 Chinese one you'll be changing it again in three months. Or maybe after the first damp day. Buy a Motorcraft one and be done with it.

With the cheap ones you'll be chasing misses from one coil to another.