Insane amount of misfires and knock on 1,3,5,7

[animuL]

We did a basic tune on my brothers 4.8L truck a while back, its mostly stock, just a few torque management things, and some timing and fueling adjustments. The truck has been running great for months but recently he says he has started to hear knock and yesterday it threw a code. So we hooked up the ol' HPT and the code was for engine misfires. Cleared the code and took the truck out. I notice a few random misfires at idle and cruising and then when he hits WOT it just instantly counts up about 100 misfires and some serious audible knock. The misfires occur mostly on #3, about 70-80 misfires on that cylinder, the rest of the 30 or so misfires are spread out across cylinder 1, 5 and 7. Does anyone have any idea what would cause this? Like I said the truck has almost no mods... just a nitrous kit that was installed over a year ago and he rarely uses it, hasn't used it in months, and of course the tune. 1,3,5,7 are all on one bank aren't they? Plugs look decent, pretty red and obvious signs of knock, but other than that they don't look fouled out. The way it pings leads me to believe its not getting fuel and thats what is causing the misfires instead of if it was getting fuel and just not igniting.

Thanks in advance for any insight!

[hirdlej]

Almost sounds to be like the wiring harness for the coilpacks may be dirty or not seated properly on the odd firing cyl bank. Does he have headers or anything that are too close on that side? Something not grounded properly?

[animuL]

No headers, and hasn't really done anything to it so its not somethingthat could have been disconnected or messed up on accident, nothing has been touched. I too thought maybe a bad ground or something that was keeping the injectors from firing.

[SMOKINV8]

How do the O2's look? If one of the O2's is going out, if the cat is clogged, or if you've got an exhaust leak near an O2 it can cause the symptoms you describe. The latter would be pinging from running too rich, and I would lean toward one of the other possibilities.

[animuL]

those were my thoughts... what works on a bank by bank basis? O2 sensors and maybe some of the wiring and of course fuel trims because of O2s. Might have to go back and check out the O2s. I suppose if one sensor was going out it could read a false rich condition and supply less fuel to that bank and cause the misfires and pining.

[ninobrn99]

you can never be too sure. check your plugs, wires, packs and connections to them all. fuses. stuff like that.

[jwright350]

swapped the rear o2 to the front (since rear is turned off) and same thing happens.... I'm leaning towards the plugged cat idea. My wiring is great (double checked it anyhow), the truck is an '03 but only has 31K on it... its showroom fresh under the hood. I think I might try to swap the o2's from left to right just to be safe and change the plugs for the heck of it.

[SMOKINV8]

Emissions stuff would still be under warranty on that one (the entire truck may be?). If not and you have a connection at a local dealership, I'd ask them just to check the cat to see if its bad using whatever techniques they use. Flash back in a stock tune before you take it in just to help them verify if the cat is or isn't the issue.

[jwright350]

i'd have to take the nitrous stuff off though... thats a pain

[jwright350]

flashed it back to stock, dealer replaced both cats and y-pipe at no charge under warranty... the had no clue it had NOS on it or that it has a tune..... runs perfect, time to start the tuning process again

[EC_Tune]

Congrats! on a good diagnosis!