So I changed my spark plugs and as part of the process I cleaned out my throttle body with a rag and no solvent. I just cleaned around the throttle plate, which I moved as part of the cleaning. My battery was connected the whole time.
Once I put everything together and had driven it a few times I got the dreaded p0507 DTC (Diagnostic Trouble Code). I googled around found that this was a common problem after cleaning the throttle body. Removing even small amounts of gunk improves the airflow, but also screws up the engine computer which has learned to expect a lower airflow. This extra airflow raises the idle RPMs (mine was 800-850) and also makes the computer think you have extra air coming in through a vacuum leak in your intake piping. Not sure why the MAF (Mass Air Flow) sensor doesn't detect the extra air coming past it though. You'd think it would detect the higher air volume flowing by it and understand it was not a leak.
I was unable to reset the IAVL, Idle Air Volume Learning, in 10 attempts. You'll see the procedure documented elsewhere on the web as well as in the DIY service manuals.
The fix that finally worked for me was to clear the codes, disconnect the electrical harnesses to both the Mass Air Flow sensor and throttle body, and then start the car up with those unplugged for maybe 5 seconds. The car will run rough and throw all kinds of code, so let the Check Engine light come back on. The turn the car off, plugged both back in, and clear the codes. All this was done with the battery connected. I found the post that taught me this here (see the very last post): http://x.nissanhelp.com/forums/altima/22429-high-idle-vacuum-leak.html
I've now driven about 200 miles since I did that a few days ago and the Check Engine light has stayed off. I'm hoping this is fixed. This was very troublesome and is a known problem.
The check engine light came back on with the same p0507 trouble code, and the idle remained high at about 800 RPM. So the fix that REALLY worked for me was to disconnect the negative battery cable for 24 hours. Upon reconnecting, my idle remained low at about 600-650 once the engine was warmed up, and no more trouble codes appeared. Hooray! I've driven about 2000 miles and no more codes have appeared.
I read about the battery disconnect fix somewhere on http://www.nissanmurano.org. I didn't need to perform the IAVL procedure again.
I think this fix worked because the ECM (Engine Control Module) runs out of juice in its internal battery over that 24 hours so it doesn't retain any memory of its prior settings, including the Idle Air Volume. Everything is erased, so it automatically starts to relearn once you restart the car.