Scott,
Why would you want to run your engine with "less coolant"? You run the risk of major engine damage (cracked head, blowing a head gasket or worse)...KEEP the radiator full!
Double check the thermostat to see is it's working (when you remove it from a cold motor it should be closed, put it in a pot of cold water and heat it up on the stove and watch it open). Also make sure it's installed correctly with flow > to > the radiator). If you run the engine without the thermostat the engine will have trouble getting up to operating temperature.
Have you tried putting a piece of cardboard in front of the radiator to block the air flow?
Mike B
