P0300 random misfire code

[RennsportRaptor]

I have had an intermittent P0300 code for random misfires for a while now. It only happens between shifts at full throttle occasionally.

I replaced all 16 spark plugs as well as coils and wires yesterday and I still have the code. The truck runs great, doesn’t have any lack of power or make any strange sounds. I’ve driven cars with misfires. This truck isn’t misfiring.

We have a smoke tester and we’re going to try that next. I’m wondering if it’s the O2 sensors, MAF, or cats. Has anyone else had one of those cause this code? It has a Roush intake and exhaust and is not tuned.

Thanks.

 

[thatJeepguy]

Is it one cylinder that has the mis or is it random?

 

[RennsportRaptor]

Is it one cylinder that has the mis or is it random?

It’s a random miss. I bought it in August. It started throwing the code in October. It has never identified a specific cylinder that is missing and I have never felt a noticeable lack of power or anything. It does drive much better with the new ignition, but not like it was struggling before.

Thats why I’m thinking it’s vacuum, MAF, or O2 related.

 

[RennsportRaptor]

Realize I never updated this.

I noticed my Roush rear-exit had a horrible, tiny, resonator welded into it. I bought a new Roush side-exit and installed it. Truck drives so much better, makes more power, and hasn’t thrown a CEL since. I didn’t really expect that to cause my issue, but it necked down from 3” pipe to maybe 1.5” or 1.75” id where it necked down to that crappy res.

Sounds better too.

I know it doesn’t look drastic on the outside - I figured I had a clogged cat with how quiet it was and that P0300 code, but when I cut this apart, it had two more pipes inside necking it down to fit this junk resonator, and it looked like the mesh was failing in it, partially blocking the exhaust. It was JUNK.