Recently bought a 2012 G37 automatic with 70k miles. Idles normal when cold. Starts at 1400 rpm and quickly warms up, rpm's sit at about 650 when in drive and still cold. But after driving it like 2 miles or idling for like 20 min the rpms go up. 

When hot and in drive, I don't even have to touch the accelerator and it slowly goes to 1600 rpm and gains speed on its own all the way to 25 ish mph. When in drive and at a stop light it sits at 950 rpm and the fuel economy is terrible in the city due to this. Getting like 12 mpg in the city. But surprisingly 27mpg on the highway. Drives and shifts perfectly fine and when I floor it, it pulls hard.

Things I have done to it in my 2 months of ownership: 

- Replaced left side air intake boot (old was torn)

- Checked spark plugs (on the original set and still good)

- engine oil and power steering oil

- Replaced fan control module because old module was bad and had fans at 100% speed all the time (new aftermarket module does work on both low and high speed)

- got the Battery load tested at autozone, asked them to scan for codes, no codes 🙁 

- tried doing the idle air relearn procedure I saw on YouTube, no noticeable changes 

It seems to have been gradually getting worse? (rpms are higher day by day) lol 

I don't want to throw parts at it haha, I don't have a real scanner but I think I might end up getting one of those cheap $5 scan tools on ebay. Temperature gauge stays at its normal position and doesn't seem to be overheating. (googling says maybe thermostat opening too slow or something but idk, feel like I really need a scanner with this car. My old 1991 Nissan Maxima was soo easy to work on 🤣)

Might end up taking it to the Nissan dealer and pray I don't get hosed and get the transmission fluid changed while I am there 😆 Infinities are rare in my State down here in Mexico and local mechanics will probably ruin my G37 due to lack of knowledge.