If the system has a sight glass, it should go bubbles and then clear then bubbles and then clear. This is a normal cycle for the Mercedes. When the aux fan is on, you will have more bubbles than not.

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Benzmac:
Donnie Drummonds
1991 GMC Syclone
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM