2000 3.4L Sudden Severe Misfire

Short version: Truck has 200K on it. Suddenly started to misfire on one cylinder, shaking at idle a little. Within 3 miles it was 3 cylinders, severe misfire, almost can't drive it. Codes 300,303,305,306. Also have a code for a bad cat (P420) which has been there for a few months. Plug wires aren't arcing as far as I can tell. I've been reading about all the standard stuff and I'm definitely going to do plugs and wires because I just bought the truck so I have no idea how old they are but the fact that this started so suddenly and immediately was severe and random/multiple cylinders makes me think plugs/wires ain't gonna cut it.

Should I switch the coils around? One coil hits two cylinders on these, right? So if I'm misfiring on 3 cylinders it seems unlikely that two coils went bad within 2 miles of each other, right?

Could this be related to 02 sensors? I know the cat code could be from the 02 sensors rather than the cat itself. Is there a way to check or do I just gotta drop the $$ and replace?

I'm a 20R guy so all this computer stuff and fuel injection is new to me.

 

All of these things are able to be tested. Various methods to test them, like you mention. Get the FSM and then you can follow definitive testing. Running with O2 sensor malfunctions will default the ECU to richen the mixture to be safe. If O2 sensor malfunction isn't addressed and you run rich for a good time you can kill the cat and the plugs prematurely.

 

Yeah, that's what I'm going to do, just hoping that someone might see this and go 'oh it's prob XXX'. If it was basic misfire stuff (misfire on one cylinder (plug or wire) or two (coil) or it's been going on for a long time and slowly got worse (plugs or wires) I wouldn't have posted anything would have done plugs/wires/swapped coils around first but this being so sudden and so severe is confusing to me. But like I said I don't know much about fuel injection

 

Look closely at the plug boots below the coils also. Essentially short pug wires and they do crack and misfire inside the well.

 

Might be worth just pulling the coils and inspecting them visually. I had a rough idle, and so went to look at the plugs. Upon inspection one of the coil packs had a big crack down the side, replacing it helped a ton.

 

To close this out, mostly I'm just another idiot asking for advice before doing the basics. I needed new plugs and wires, I also replaced the MAF sensor. I got a noticeable improvement after I cleaned the MAF and then when I replaced it that cleared up the misfire pretty much except for at idle. Then new plugs and wires fixed the idle. I'm still confused about why the computer was so suddenly giving me a misfire on three cylinders but I think I was just over reading it, it was probably mostly on #6 which was the worst and just enough on the other two to throw a code. Coils are fine.

 

Awesome man, appreciate you taking the time to report back! Glad it was an easy fix for ya. Not always, but many times we get pretty lucky with these tacos and how easy they are to maintain, and to fix when an issue crops up which is rare In and of itself.