Guys, a bad plenum isn't going to cause a single cylinder misfire unless it fouls up one of the plugs (unlikely.)

You're on the right track with a fuel injector.

 

what clips are you talking rusting and breaking off causing it to misfire in the 8th cylinder?

 

4 year old thread. I am hoping he has this fixed....

 

I just got a misfire on cylinder 8 last night, exactly a week after doing my plenum. Plugs are new autolites, and cap & rotor are about two years old(20k mi). Swapped out injectors 8 & 6 and cleared the CEL. Idle is a little rough, you can feel the miss. Did notice that injector 8 is missing an o ring on the end that goes into the intake as well as the plastic nozzle looking thing that goes on the tip... Waiting for CEL to pop back up and see if the misfire fallowed the injector. Any other suggestions?

 

The missing injector o-ring would make it run lean. That might have been the whole problem.

 

Well my CEL hasn't turned on again. But the miss feels different than before. I think that it's safe to assume that the injector is the problem... Right?

 

Maybe.

Start the engine, with it running at idle, can you hear/feel the miss? If so, disconnect the injector in question, does anything change?

 

I know that post is old, but my plenum issue presented itself with #8 cylinder misfire from fouled spark plug...

 

I switched 8 & 6 back and bought the o ring for #8. Smoothed out a good bit. Idles nicely at 650rpm. Does seem to still have some kind of miss or something. When I let off the brake and let it idle slowly it seems to miss or something, the truck shakes a bit. But at a stand still it's fine. Any ideas?

 

At idle, it may simply be a lean-burn miss. Does it do it right away? Or, only after running for a few minutes?