In extremely cold weather, very cold air flowing onto the radiator and through the engine compartment can prevent the engine from warming up (and also keep your heater from working well). Radiator shutters or a winter front restrict this air flow to let the engine warm up. However, don't overdo it. Keep the winter front partially open or keep the radiator shutters free from ice. If the shutters freeze shut or the winter front is closed too tightly, the engine may overheat from lack of sufficient air.