Just replaced coolant (Benz coolant) on my 91 300E and thought I had everything in order. However, I guess I missed something. Now when I start the car and the climate control dial is set to 80 I hear sound of coolant gushing near the dashboard. The sound stops after a while (under one minute). This makes me think that I have air in the system.

I squeezed the hoses, left the radiator cap open while blasting the heater in full mode (the coolant does not seem to rise in the overfill tank while the overflow cap is open) and still I hear this gargling sound by the dashboard.

I read in the archives that there is a vent screw that also must be opened to allow air to escape while pouring the coolant. I can't seem to locate this vent. Can anyone point out its location. Also, why is the coolant level not rising in the overflow tank with the engine running and the heater in full blast mode. I hope that I have not screwed up.