i have a 323i will not idle check engine light is on

Will need more information about the issue, have you pulled fault codes? Does it run ok with your foot on the throttle?

Agreed. It could be the idle (air) control valve if the engine runs with the gas pedal pressed, but you may want to go to Autozone to have the codes read to be sure - they perform this service free there, but call first.

A friend had the same problem...air filter was junk. He replaced with a K & N unit and it fixed the problem.

Regarding the K&N filters, DO NOT use these on cars with mass airflow sensor that can easily be contaminated by grease (used in all K&N filters)! Most good quality oem air filters has equivalent flow rate and generally provide better filtration but with no risk of contamination.