Hey guys, it seems like I have one cylinder that likes to destroy spark plugs. I swapped in new plugs/coils a few months ago and that fixed the misfire issue I had when I bought the car. I also had the vanos solenoid replaced so thats only a few thousand miles old as well. Then about 3 weeks ago I swapped in these plugs because I was getting the car tuned and after a few weeks the car began (sometimes) starting up oddly when cold or sitting. It would start up very erratically and then smooth out within a few seconds. The engine light finally came on after awhile and gave me a P0303 code for misfire cylinder 3. First thing I thought to do was check the plugs so I did and the plug on cylinder 3 seems like its been through it. I also have a comparison to the plug on cylinder 2 that looks better. What could be the cause? Im planning on getting my intake valves cleaned could that cylinders valve be extra bad? I believe the misfire I originally fixed was also cylinder 3, however the car had other issues at the time so it would sometimes misfire other places or idle badly. The issue is ONLY on cold start, it drives and idles and everything else perfectly. Oil level is fine and was changed within 1,000 miles. Everything else I can see visually seems fine as well. I went and exchanged the bad plug with a new one today and it’s been fine since, but why is it messing up plugs there? Oddly enough as well, when I first ever tried to unscrew the plug in cyl 3 the threads are a bit bad, other cylinders it comes right out, three its tougher to turn to loosen or tighten. four is similar but not as bad.

cyl 3

cyl 2

 

Try swapping the cyl 2 & 3 coils to see if the problem follows coil 3.

 

i actually tried this and instead of the problem moving it went away entirely the next morning lol
but it came back on subsequent startups. ill try to start up the car now and see if my scanner shows live data on where the misfire is happening if it still is at all after the new plug. i could also swap back in my oem coils, im using bosch ones now.

 

What engine are we discussing? N12, N14, N16, or N18?

Intermittent misfires are difficult to track down, and normally requires a skilled tech who knows how to use a lab scope. In your case if you have one cylinder that is fouling a plug (your cyl 3), I would suspect a problem with the piston rings or valve train, but you indicated the engine drives and idles perfectly which would tell me to look at other things besides mechanical engine damage. You may have an intermittent fuel injector, but you'd need a lab scope to see that too. The plug in your pictures looks normal to me.

Keep in mind the DME will shut off the fuel injector for cylinder 3 when it sets the P0303 DTC to save the catalytic converter.

 

sorry forgot to say its an n14. im also thinking an injector but youre right most of these things id need a scope etc to see so ill probably take it in for a diagnosis if the problem keeps coming up. i got the valves cleaned today so i will see if on cold start etc it has the issues. so far it feels noticeably better. both plugs look normal to you? the top plug doesnt look like it had extra wear etc at the top? or like maybe some oil is getting onto that plug? the coils are dry never oily so im not sure.
heres a better picture

 

Yea, that plug looks fine. However, I've see perfectly looking plugs that were defective internally.

You mentioned you had the engine tuned. Exactly what work was performed? Did you change the spark plugs to a version with a different heat range to accommodate the tune? Can you remove the tune and try your cold start again?

 

i figure ill drop an update because it seems ive fixed the issue. i put in a brand new spark plug on the problem cylinder and got the car walnut blasted but neither permanently fixed it up. it does drive better after the blast etc but still. i then replaced the valve cover because the one I bought the car with was totally failed. leaking all over the pcv. it mustve also been allowing oil down into the spark plug hole on cylinder 3. since theres a brand new plug in there at the same time as the valve cover change ill pull it eventually and check if it looks worse than the other plugs, but for now it seems its solved. cold/warm it starts up smooth with no problems and no codes.