On a cold start, my 05 will high idle normally at about 1300 rpm for about 5 seconds. It will then slow a little bit to about 1100 rpm and develop a shake as if it is not firing on one cylinder. Within a few seconds more the idle smooths out to about 800 rpm and runs smoothly. A little longer and it settles into curb idle of 600 rpm. I have noted a strong acetone-like odor to the exhaust when this shaking issue occurs.

My OBD reader has logged no faults or messages.

Does anyone have any suggestions?

 

This may be a reflash issue. There is a Dealer procedure to correct rough idle during start up. I had it done some years ago. If there were a mechanical problem, it would not smooth out so it must be software.

 

I've had an 04 X350 and my current 05 X350 and both did this same thing. Its pretty common and not really anything to worry about.

 

Thanks for your thoughts gentlemen. I'll inquire with my indie mechanic next time I see him.