Every once in a while within 20 seconds of starting my truck it'll throw a p0304 (cyl. 4 missfire). This can go several months with out occuring but recently happened twice in the past 2 weeks. I had this problem for some time now and the first time it cropped up I replaced the coilpack on that cylinder and thought that should do it (being the plugs were basically brand new). A couple months later it came up again, cleared it, a few months later it came up again.


Being that OBD II detects a "missfire" as the plug didn't fire I'm trying to figure out what else might be the cause here. What exactly are the coils hooked up to? Right to the ECU? Or is there some separate ignition control unit?

I do have a superchips 91 tune and that advances the spark timing a little so I don't know if that might play a part at all but i still think it's really weird cause it only sometimes happens.

If there isn't a separate ignition controller i feel like the only other relevant part would be the wires. I'd be surprised if the wire was shot though - or at least expect a more consistent problem.

 

Doesn't ever hurt to do plugs and wires!
I've heard of this happening on 87 grade fuel, maybe you're getting a batch of bad gas occasionally? I know in Canada fuel quality varies greatly between stations.

 

I've been running strictly 93 from quik fill or mobil or sunoco. The plugs have maybe 10k on them at this point (ngk v-powers), but I'm guessing the wires might be stock - which would mean around 40k on them. Kinda weird that it's only so occasional though.

Also want to mention that I clear it right away and it won't show up again even on the same tank within the remaining 300 or so miles.

 

I was gonna suggest swappin coilpacks with another cylinder to see if the error missfire issue follows it...but you said you switched coilpack out, next suggestion is either new wires or swapping wires..