I've been having some problems with my 03 g35 sedan. Just recently hit 100k. It started off slow. One day I notice the car was shaking and very rough at idle. Drove for 10 min and it didn't happen for another week. Then it did it again but went away. Now I'm to the point that it's non stop shaking idle. I will start my car at a cold start and it doesn't shake. Once the rpms drop bellow 1000 it starts to shake. When the car is completely warmed up it sits at 700 rpms but since the shaking started it fluctuates from 700 to 600 and the whole car shakes. I've taken it to firstone and they told me it was a simple fix. Told me it was going to by throttle body. They quoted me 1000 but I went ahead and bought one from Nissan and replaced it myself. This did not help at all. I did the idle air relearn procedure multiple times. I recently ordered some coil packs. Just because the car is 100k and wanted to replace them before they fail on me. The one thing that bugs me is that it runs fine when it's warming up and the rpms aren't low but once it warms up it's rough. Seems like a sensor isn't sending a signal to the engine that it needs less gas or something. It's a very rich idle. The exhaust smells strong and it rumbles from the engine. No misfire codes. I'm thinking o2 sensor maybe. Don't think it's cps sensors because the car runs fine when driving just very rich and shaky at idle. Any suggestions? I've been searching many threads for an answer and have had no luck. Will post a video of how the car sounds if wanted. Thanks in advance!
Jake

 

Scan the car for trouble codes. It might be a cam position sensor which can start to fail, not trigger the SES light but throw a code. Symptoms you are having are classic CPS symptoms. At least this'll confirm or rule out 1 possibility.

 

Vacuum leak.