Why the light?
 

2003 X5 3.0i w/82K miles. Runs great. Started today after sitting in basement garage for three days. Check engine soon light came on and idle was terrible. Drove to close-by car wash and then home. Felt like running on four cylinders only; very rough and zero power. The higher the RPM's the worse the engine vibration.:yikes:

Pulled into garage, turned key off, restarted and ZERO issues now. Runs like before--just about perfect. Wonder why I got a check engine soon light? Now it runs fine.:dunno:

The funny thing is that I'm trading it tomorrow as I made the deal right before Christmas and they needed to get the new car ready. Was my X5 just pissed at me or what????:(

battery may have been low from sitting.

I would say your Mass Air Sensor is going bad. Try starting it plugged off in cold. Bad idle should go off...

Plugged off?
 

Battery is brand new; about one month. Starts engine fine.

Plugged off meaning unplugged sensor? Restarted and it ran fine then. Will see in the a.m. what happens.

Was your SES light blinking during the rough idle time?

No blinking light
 

No, there were no other warning lights on the dash other than the check engine soon light. Weird, eh?

the SES light is service engine soon, i believe the same as check engine soon? you'll have to excuse my possible misuse of nomenclature i'm a week old E53 owner so i'm still learning. I just saw a thread recently about rough idling on cold start and the SES light blinking (not solid) which i believe indicated cylinder misfire.