My 99 z28 A4 (50k miles) idles at 600rpms when cold and purrs like a kitty, but once its warm and the idle drops down to 500rpm-490rpm it feels lumpy, enough to cause a moderate vibration as well.

Could the spark plugs be going out?

 

Idle is too low. Should be around 700-750 for RPM for a M6 higher for an A4 out of gear. Thats why it is smoother when cold because it's idling higher.

 

IAC is probably acting up or passages are dirty.

 

when your engine warms up to operating temp, the fuel system enters closed loop and the 02's take over your injection. check your 02's and LT trims.

 

iac? LT trim?

how can i check for a bad O2 sensor?

appreciate the response guys

 

Idle Air Control.............on the throttle body

Long Term fuel trim..........a function of the PCM

Hook up a scan tool and watch them cycle.