So i just bought my first g35, its a manual coupe and after about a week of daily use check engine light turned on. Upon scanning it got a random missfire code p0300. Initially i thought it would just need new spark plugs, coils etc. after changing the oil and taking off the spark plugs off, they were still in good condition expect for the cylinder 6 one which has oil around it and on the coil tubes. Could this be what is causing the missfire? How should i go about this? Should i replace the valve cover or just the ignition coils?

 

Yes - new valve cover with oem gaskets (or you can reuse old with dremel out spark plug tube seals).

 

I have had the oil on the spark plugs twice on my vehicle, neither time did it cause a misfire and I had oil on 4 of the 6 spark plugs. I am not saying that it can not or is not the source of your misfire, but I am saying it LIKELY is not the source of your misfire.

P0300 is a random misfire, if it was due to the oil (and you only have oil on #6 plug) you should see P0306. I would start with replacing the cam and crank sensors with OEM Hitachi brand. This is not a waste of money as you will have to do this job one day eventually. When I got my P0300 I decided to do the cam/crank sensors since it was a matter of time until I got those specific codes, and I haven't had a single misfire since. If you still have a misfire, continue inspecting the plugs/coils before you decide to replace the valve covers.

That being said, Urbanengineer is partly correct and the only way you can get rid of oil pooling in the spark plug tubes (and inevitably making its way to the combustion chamber) is by replacing the entire plastic valve covers. I say partly correct because our vehicles do not have serviceable spark plug tube seals, they are embedded into the valve covers. I had replaced my valve covers back in 2018 with a cheap brand off rockauto, and 4 months later when I did a compression test, 4/6 of the spark plug tubes were FULL of oil. I drove like this for 3 years with no misfires, and god knows how much oil I was burning.

Frustrated with how much money/time I had already spent, I used a hole saw to drill out the tube seals and see if I can place new ones inside. No dice. I then spent the ~650 CAD on two brand new valve covers from Nissan, and I even put a thick layer of RTV on the outside of the spark plug tubes as well as a thin layer on the inner perimeter of the seals within the valve cover, and reinstalled. Torqued everything to spec as per the FSM, waited the 24 hours before starting the vehicle, and guess what - when I did another compression test this past summer, I once again have oil leaking into 3 of the spark plug tubes. I hate our valve covers.

 

You could be correct, when i bought the car guy said he had it sitting for a while. I will start with replacing the camshaft and crank sensors first. Had another friend tell me the same thing. If thats not the issue i will move on to the ignition coils and plugs etc. I’ll keep it updated in case i need any more help. Thanks for the feedback.