Change your condenser to a parrallel flow if you want the 134a to cool the car...
Personally I'd stick to R12 but here we go again with another R12 / R134a / 409 / Freeze- whatever / propane debate...

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Over 21 years I owned several:
w108
w110
w111
w115
w116
w123
w124
w126
w212