I recently bought a G35 and it has been misfiring. I changed all the spark plugs and it fixed the problem, but it came back pretty quick. One spark plug (#3) in particular was covered in what seemed to be oil and gunked up a lot. I checked that plug (I believe its the 3rd cylinder) again today and the new plug was dirty again. I reset my ecu and put an older plug into it that was clean and the misfire went away again. But another thing I noticed is that if I unplug that coil pack while the car is running, the idle doesn't even drop. But at the moment it doesn't seem to be misfiring (the car is pulling harder again, not sputtering, or rocking). So why doesn't the idle drop? Also, another thing to note is that it seems that someone had rewired the wires to the coil pack and the one by it. There's a blue connector coming from the wire harness and attaching to another blue connector which branches out to the coil packs #1 and #3. If I unplug the #1 coil pack the idle plummets. But #3 has no noticeable affect.

Here's a picture of the spark plug

 

Idle doesn't drop because that cylinder is probably not firing when it's gunked up like that.

It's probably not firing at all and you are driving on 5 cylinders. I would also guess the rewiring was either the owners attempt to fix the issue, which really looks like it was caused by a very bad oil leak (valve seals) into the cylinder fouling the plug.

Is your check engine light on? What codes are you getting? General misfire code or does it identify cylinder #3?

 

Well yeah, but after putting a clean plug into the car it doesn't feel as though its misfiring (no sputtering when driving and no gas smell) but it still doesn't drop the idle so it confuses me. I didn't clean the "gunk" out of the plug tube after changing it so I thought maybe that why was it got so dirty again.

I'll post pictures of the wiring later. How much do you think it would cost me to fix if that is the problem? Dad and I were talking about taking it to get a compression test done.

And yeah, it was throwing p0300 before. I'm assuming it still is but I haven't had them read since I changed the plugs.

 

Here is a photo of my plugs with 106k miles on them from my previous 06 G35


Did you ever test for spark from the #3 coil pack?
That much oil on a plug is certainly not normal

Telcoman

 

How would I test for a spark?

 

When you remove the wire from the coil to the spark-plug, use a screwdriver from the wire and put the end of the screwdriver very close to a ground and see if a spark jumps to the ground.

Do the test first from one of the other cylinders so you can see the spark jump

If you do not see a spark jump from #3 then you have a defective or intermittent coil pack
Try swapping from another cylinder to confirm

Telcoman

 

Unplug the Coil and take a spare spark plug and plug it in. Now lay it down in a way that the metal part of the plug grounds out against something metal on the engine, or lay a screwdriver across the plug to something metal.

Stand back (getting zapped by an ignition HURTS! Trust me I've done it) and then start the engine.

You should see the plug sparking away. helps to not do it in bright light.