05 6mt 110,000 miles
This started as a come and go problem. Go out car would be running fine, spin the RPM's up to the higher side and end up with the car running rough as hell. Let the sit overnight and would be fine in the morning. Has done this for the last 2 months. About two weeks ago it has done it and not gone away. There are no codes coming up or pending.
1st thing I did was replaced the plugs (NGK's) they where old and worn but not that bad.
2nd Pulled both value timing solenoids. Both where clean and moved freely hooked up to a 9volt bat.
3ed I have checked for vacuum leaks the old school way, can of carb cleaner and sprayed away and listened for a jump in rpm. Not a thing.
4th Gave up on my 200 buck scan tool and took it to a friends shop. He Put one of is guys and a 4 grand snap-on computer on it for over a hour. Tested all the coils on some sort of scoop that said they where all doing their job well. Same for the injectors. Walked away from that with "well it's not electrical maybe a bad timing chain" Not going to spend the money on that maybe and have it not be it.
5th Have done the "Idle air volume learning procedure" and reset the maf, no help.
Car has been well maintained over the years. As for the timing chain, I have had them go bad on other cars. Never had one be bad one day and good the next till it went all the way. Also car runs rough as hell but not popping back though the intake or backfiring.
Any help would be great. Sick of driving a G35 that feels like a Harley! Thanks a ton Dave

 

I think I read the other day about someone having similar problems in the high end RPMs as well. I think one of their cats ended up being clogged up or something.

You could also check the alpha rating on your air to fuel ratio. I just did that the other day and found out my MAF is faulty even though it wasn't throwing any codes. It was maxed out at 135%. I also recently just replaced my timing chain myself and it did help my car run a lot smoother. You might also want to bring it into an Nissan/Infiniti or indy who has a Consult device. I bought a Chinese clone myself that works fine, but it picked up a lot of things that my other scanner didn't. How does it idle?

 

Idle is like crap. Runs rough across the entire rpm range. When I had it down to my friends shop on his computer (much more then just a scanner) I'm sure the maf was one of the things they checked. But I can hook up my scan tool and get a "live" read off of it to be sure.
There is no noise coming from the front of the engine that would suggest there being any kind of slack in the timing chain. That with only 110k on it with oil changes every 5k I have a real hard time with the the though that the chain could be bad.

 

I have almost 128K on my car and had a bad water pump. The timing jumped while I did that, so that's why I ended up going in there in the first place. But, it did help my car out since I had a couple of worn guide rails and a little bit of chain rattle.

This is the check an Infiniti/Nissan tech had me do to check my MAF. Let the car fully warm up and then check the Air to Fuel Alpha Ratio setting. It should read 100% +- 9 or 10%, so basically between 90-110%. Then shut the car down, unplug the MAF, and restart the car. The Air to Fuel Alpha Ratio should default down to 75% and occasionally try to set itself back to 100%. If your MAF doesn't drop down to 75% check for an air leak somewhere.

Does it idle in the correct range 650+-50 (maybe the relearn procedure didn't go through)? Maybe hit the MAF with some of that CRC MAF cleaner see if that helps as well. How about your air filter? Another thing I did was put some SeaFoam in my car (not to clean it), but to help find any leaks. That's how I found out I needed a new Y-Pipe and had bunch of exhaust leaks near the flanges. My car really ran like **** before I fixed the exhaust problems at idle, driving, and pretty much everything.