Fuel mixture becomes richer at idle

I have a 1990 560 SEL with a mystery fuel enrichment problem. Using the fuel analyzer on the X11 connection, fuel enrichment progressively increases at idle. If I hit the throttle it goes back to normal (45%), but gradually gets richer as soon as the engine drops to idle. The car stinks, and my mileage sucks. My mechanic is a Master MBZ mechanic with over 25 years experience, and he has never seen this behavior. Does anyone have any ideas?
Thanks, -rha