Summit has an Accel tune-up kit that includes spark plugs, wires, cap, and rotor, for about $90-ish. It did wonders for my truck - it looked like the plugs had not been replaced in a decade.

I pulled the boots off each plug and labeled them with a Sharpie as I went, and then pulled the cap off with all the wires attached. I then matched the new ones up (the Accel kit comes with them cut to length) on the new cap and labeled the new ones too. Then put the new rotor on and fed the wires through and put the cap on.

 

A tune up isn't going to do jack in this case, LOL

 

I know it was a cyl 8 misfire, but it COULD have just been something non-plenum related...

Or the plenum could be beyond bad.

Or that could have absolutely nothing with the shaking at all, and be another issue by itself.


And like someone else said, it could have something to do with one rotor being turned and one being replaced. Kind of doubtful but possible.

 

#8 is the most common place for the pan to leak,and it's closest to the cat, so the extra heat from the cat getting clogged, takes it's toll on that cylinder.

 

FINAL SOLUTION: It was just the spark plug. My mechanic replaced that plug and now it drives smoother than before. $6.50 for the plug, $10 for the install. Thanks to everyone who chimed in.

 

OK, you'll see.

 

I understand that cyl 8 is often a representation of a bad cat. But if it were me, I would have done what he did, put a plug (or wire, whatever) in and seen if it fixed it. If it does it again in a brief period of time, yeah, it's probably a bad cat. If it doesn't do anything, it could just be a plug, wire, etc. that had gone bad.

My point is only that before spending the money on a new cat, I'd rather replace a spark plug or whatever beforehand to see if that was the only issue.

OP: Was that the cause of the shaking as well?

 

Well, a misfiring plug might cause roughness that could be described as "shaking". I'm glad the problem is resolved. That being said... you should still go ahead with changing all the plugs, cap, rotor, and wires. If the plugs have been in long enough for one to go bad and send a code, (or if you've had the truck for a while and know they haven't been changed in years) changes are, they all need attention.

If the plugs are not too old, zman17 is right, it's a symptom of other issues. I wouldn't ignore it.