I just had a bunch of work done in my 2007 M35 with 150k. First major repairs in 11 years of owning it.

One issue that has not been resolved..

At idle the RPM's do not stay stable. I understand that 650 is "normal" but mine will fluctuate from 680 to 650 to 638 continually. A/C off, radio off, nothing else on.

Just replaced an upstream oxygen sensor that was detected from codes being reported.

The throttle body and parts around it were cleaned. Cracked hoses and parts replaced.

Fuel economy is down a bit.

I did the idle reset/relearn procedure a number of times.

Same results.

Any suggestions would be great. I know it's not a major issue but it just bugs the heck out of me.

Thanks

The fact that the TB was touched and the parts around it to me suggests you have an air leak somewhere. Maybe something didn't seat properly?

That being said, 638-680 is like super duper duper minor. Most people probably wouldn't even hear the difference. If it was like 600 to 1000 back to 800 and back to 1000 then that's different.