Interesting enough, I am dealing with a chebby that has somewat of a similar symptom. It has a dead condensor fan, making the pressures spike in to the stratosphere.

Is your shroud in place? I bet it is, but worth asking.

You cant add much freon because your car cannot disperse the heat quick enough resulting in higher pressures. Also, I read somewhere in this thread that you put the can in chilled water? The can will tolerate a significant amout of heat. I usually wedge it near the radiator hose. The warmer the gas=more gas boiling=more pressure.

Having working recirc will help with cooling the cabin while driving. Do you have your windows tinted?

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Eugene

10 E63 AMG
93 300te 4matic
07 BMW X3
14 Ford F-150 Fx2