Bought my grand cherokee new and pretty much as far back as I can remember, whenever I'm on the freeway/highway and I go uphill into the mountains, I'll start getting misfiring in cylinder 8 (OBD2 code P0308). If I downshift while it's doing it, it stops (either because loss of power eventually forces the downshift or if I floor it to trigger the downshift). That's the only scenario where I've ever encountered this. Only on the highway/freeway (i.e. high gear), only up hill, and it's consistent. Obviously not the plug or wire if it's been happening for at least 15 years if not longer! It's never bugged me that much because it was an uncommon scenario for me, but I've been going into the mountains a lot more in the last couple years so now I'd like to figure it out. Any ideas? Injector maybe?

 

Welcome to the forum.

Has the spark plug wire routing TSB been implemented?

 

I wasn't aware of that. I'll give that a try for sure! Thanks for pointing me to it. I'll report back sometime in the coming weeks after I've had the chance to see if that makes a difference.