Alright to start off my plenum is blown I got the rebuilt one ready to go in just waiting for my day off on wednesday and hopefully its nice here in Ohio. Today I changed my ignition coil and degreased the motor had oil from leaking valve covers which I just did replaced yesterday. When I started it though it was missing more than usual so I pulled the codes and now i'm getting 6 & 8 and I was just getting 8 which I know is plenum (catalytic converter) related. Can the plenum cause a 6 & 8 misfire though?

Note : I pulled wires one at a time and blew out with a compressor helped a little but still missing.

Thanks in advance you guys always got great advice. Attached pic of intake I got it from junkyard sanded primed and painted in high temp paint new gaskets installed and all new sensors paired with a brand new mangnaflow cat.

 

So your cat is clogged? I would fix that first. Double check plug wiring and TSB on routing.

 

Just checked plug routing its right, and I don't think its clogged this miss comes and goes somedays it doesn't do it at all others it does it really bad but it was only 8 and now its 6 & 8.

 

What do plugs look like? How is your fuel pressure? Maybe swap those injectors with others (label and track where you swap them) and see if problem follows.

 

plugs I replaced last june when I bought the truck wires and brand new as of a month ago summit performance and the cap and rotor are new as well from summit. Correct me if i'm wrong but if that cat was clogged it was always miss not off and on right?

 

Also not sure if this eliminates anything but I was told you check if an injector is good by checking resistance, all injectors read anywhere from 12.5 thru 13 ohms so all electronics are operable so the injector is functioning I guess it could be clogged?

 

Injectors might be a good place to start....also you could have bent a valve.

 

Would a bent valve be intermittent though?

 

The fact that plugs were new in June or even in January isn't the concern. What do they look like now? Wet, black, light brown, etc. I assume you were not getting these misfires in June, but you are now. So their condition now is relavent.

Cat may functrion differently when cold vs hot, so it is possible that it could be intermittant.

Electrical values of resistance in injectors is important, but also as important is if fuel can flow through it. Sometimes injectors get clogged.

Another easy check would be to pull coil wire, then pull the valve cover off and have someone crank engine around. Look for broken valve springs.

 

No in june I did have the misfire but it was very slight before and after plugs it has slowly gotten worse, I can pull the plugs wed when I hopefully do the intake. Then i'll be taking it in to have the new cat put on. I also have a new set of injectors on the intake I rebuilt so if it was an injector i'll know also just had the covers off I looked over everyone of the valves while I was cleaning the head off nothing looked broken no sludge build up nothing just light brown.