3800 ft is far from "high altitude". I'd be surprised if there's really any effect from atmospheric pressure. Perhaps the difference is humidity or temperature related. I'd expect an atmospheric issue to affect power more than idle. The turbo cars with the ALDA have virtually zero effect on fueling at idle or low throttle inputs. The ALDA is there to prevent smoking until the turbo spools, then enrich fuel to match the boost pressure. The ADA is to help reduce fueling at higher altitudes for the same reason.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]