check to make sure that the aux fan(s) are coming on low stage. When the car is idling in hot weather with the A/C on it shouldn't take too long for the refridgerant pressure/temperature (not sure what the sensor on the 190 is) to turn the low stage fans on. This will blow air through the condensor allowing the refidgerant to do its job better.

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Daily Driver: 02 E430 4MATIC
In the family: '03 E500 // '04 ML500 // 64 220SE