I was driving from ATL to South Carolina the other day, had the cruise control on, doing about 75. The engine stumbled like it was misfiring and the SEL flashed and then went away. I had my code reader so I pulled the code P0304 - Misfire on Cylinder #4. It didn't happen after that so I kind of chalked it up to bad gas or god knows what. On the way home it happened one more time, so I decided to start simple and change my plugs, which I was due for anyhow.

Last night I put in NGK platinum plugs all the way around. Cylinder #3 had a little bit of oil in it, but it wasn't too bad. Might have to do that valve cover sooner or later but I'm not too worried right now. It felt like it wasn't as tight as the others, so maybe it just needed to be torqued down? We'll see.

Anyhow, I inspected the old #4 plug and it looked normal and the ignition coil also looked fine visually (doesn't mean much, I know). I finished the job and took the car for a spin. Everything was perfect.

This morning I was heading to work, got halfway in and got the same hesitation and misfiring. It was much more consistent than it had been the first time and didn't go away like it did before. Checked the code and now it's a P0301 - cylinder #1 misfire. I did one plug at a time, so I know nothing is out of order there. When I get home, I'm going to pull #1 and check the plug, gap, connector, etc.

Any other ideas about why the misfire would have moved? And why would it run great last night?

EDIT - Bone Stock G35. No mods (if that helps eliminate anything)

 

UPDATE - limped home form work (misfired all the way home) and tore into the #1 plug again.

The gap was set at .028 on the #1 plug. There are NGK 5018's, right off the shelf from AutoZone (and correctly listed on NGK's site as being for this application). I don't think I've ever seen a plug gapped incorrectly (not that I do that many plugs) so I stopped checking them a while back! I set the gap to .044, which should be the proper gap, and put everything back together. It's running like a champ now, so we'll see how it holds up over time.

I guess at some point I need to make a point to go back and check the gap on the other plugs as well, but I'm kind of thinking if it ain't broke, don't fix it. I guess it's possible that the gap wasn't the problem and I just had a bad connection on the coil, but I checked it over before tearing into it, and it all looked good. Who knows?