idling with ac on is not really a good test because the engine is in a steady state. with the aux fan on at low speed the heat is being dumped by the radiator adequately. i would be more interested in heavy stop and go or prolonged cruising followed by stop and go. this really loads up the cooling system and is a better test...
1993 300e-2.8 - gone now <sigh>
"Do not adjust your mind, it's reality that's malfunctioning"