Coolant or water doesnot make this much difference in temperature.
Possibility:
The compressor might be putting load on engine due to a jam compressor bearing.
If the car on auto fan, thn check whether the fan starts when u turn on the ac or not. if the fan is direct, how is the suction of fan?
Might be possible that the compressor is not tripping due to the thermostat switch of ac.
please post pics or your car, radiator condition and tell what did the ac wala done.
