I would leave it on. The engine is cooled by the radiator/coolant in a pressurized closed loop system. I don't think the temperature will be affected much by external air circulation unless more air passes through the radiator. Even then the thermostat will keep it near optimal temperature unless it is extremely hot and you are loading the motor extensively like going up a mountain pass @90mph with the A.C. blasting and your 400# mother in law riding shotgun.