Hi all,
Long time lurker - finally had to make a post after this issue I've been having since March.

My G35 Coupe has this rough idle going on since March, where the exhaust sounds like its sputtering. If I give it gas in park, it will also do this sputtering as well. However, when I am driving it, it is not noticeable. The exhaust smells a little rich, and it is throwing a pending P0306 (6th cylinder Misfire) code. The issue disappeared for a day when I did the oil change, and also goes away if I take it for a spirited drive.
I have replaced:
- EVAP Canister Purge Valve
- Rocker covers on both sides
- Ignition Coil
- Spark Plug
- PCV Valve

I have also ensured all vacuum hoses are on properly and that there are no intake issues. I cleaned the throttle body and did an idle relearn as well.
I have attached a diagnostic report from an OBD software to this post. Below are the videos I took, one from the engine bay (by Cyl 6) and one from underneath the car, under the rear muffler:

Any ideas on what the issue might be?

 

What's the mileage on the car? Still on original engine?

Is the air filter clean? MAF sensor?

If you've changed the coils and you're sure coil 6 isn't defective or DOA it may be time to do a compression test on #6...

 

256,000km, original engine.
Air filter is clean, not sure about MAF sensor.

Is a compression test a DIY thing or should I take it to a shop?

 

256,000km is getting up there in age but these run 300k+.

What was the condition of the spark plugs, especially #6? Any really black ones? That could be an injector sticking.

Back to the coil packs, are you sure the connectors are on the right coils? Easy to connect #4 to #6 and vice versa. Also try disconnecting #6 while the car is running and see if there is any change. If not, coil pack is the problem.

Recheck all vacuum lines and fittings again.

Compression test is not difficult but you will need a compression tester and #6 isn't exactly easy to work on. Maybe let a shop do it

 

The plug on #6 was a bit dirty. The threads had soot near the tip (about 1cm of thread was black) of the plug and the Electrode was a little crusty (brownish/whiteish crust on it). I cleaned the crust off and wiped off the soot and it was working fine. The coil boot had no oil on it at all.
I tried to run the engine with the coil pack unplugged and it ran much worse (sounded like pots and pans banging), so it sounds like the plug and coil were working. I also have not had coils on cylinder 4 and 6 disconnected at the same time, so I don't think they're mixed up.

Are there any fittings with common issues that I should be checking? I couldn't find any ones with issues when I checked.

Do injector cleaners help with unsticking injectors?

 

I would try a small bottle of Techron but I doubt you can find it in Canada (assuming you are in Canada, you quoted mileage in km). Gumout Regane is also good and contains PEA, CDN tire has it. I doubt this is the issue but worth a try if an injector is sticking.

At this point I would do a compression test on #6 to verify the cylinder is still strong and not low on compression.

I'm sure poster cleric670 the VQ king will give you his opinions when he sees this thread. Always very helpful an knows his stuff.

 

I'll try Gumout and see what's going on with it. If that doesn't work, I'll have to take it to a mechanic and get a compression test.

 

Easy way to hear if something gross is wrong with your injector is to hold a long screwdriver or stethoscope on the injector and stick the handle against your ear. If you don't hear the same kind of clicking from #6 as you do on the rest of your injectors it may tell you something. If you don't hear the same clicking, give the injector a tap with the screwdriver. I corrected a cylinder misfire in another car by doing this.

 

I don't think there's a way to do this without removing the plenum - no?

 

I think you have to remove the plenum to get a look at the injectors, not that I've done it but it isn't that difficult. People do it all the time for plenum spacers and MREV manifolds. Looks like there are lots of bolts to remove and the intake tube. Several videos online for 350Z

 

yeah maybe so. i haven't tried it on this car so don't know if you can or can't but the plenum gets in the way of stuff pretty good.

 

So I went out and bought a MAF cleaner today. It was quite an adventure.
The MAF on these cars is in place with 2 Security Torx screws. That took me an hour to get out.

Anyways, cleaned the MAF, and I'm still having issues

I noticed that at steady RPMs, there is no misfire. However, as soon as the RPMs climb or drop, the misfire occurs. I'm starting to more think that its the injectors...

 

Any updates here?

 

I've bought a bottle of Gumout but haven't put it in yet because I can't drive much right now. My arm has been in a sling from an injury since a few weeks ago. Hopefully I can take it off by Monday.
However, while selling my old working ignition coil to a mechanic in a parking lot, he mentioned that it sounds like I do in fact have an exhaust leak and that there are tears in both flex joints of my Y-pipe. Perhaps a bad Y-pipe and/or clogged cat might be the answer?

I think the passenger upper A-arm is also due for replacement - something on that side is starting to clunk around and creak ever since I changed the driver side. I also need to change the passenger side sway bar front bushing (I have the part), but I don't think my shoulder will be fully healed/regained strength for me to change it myself before winter. I'm gonna have to take it to the mechanic

 

Oh boy, talk about a bad run. Hope you get everything sorted out.