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1. A restriction in a radiator will usually cause the engine to overheat when at high speeds like on the freeway. You can sometimes feel the top and bottom of the radiator when the engine is at normal operating temperature and feel a drastic temperature change from top verses bottom of the radiator if restricted. Flushing will not help either. 2. A bad thermostat will cause the engine to over heat at any speed 3. A bad electric cooling fan motor will usually cause the engine to over heat at slow speeds or while at a stop 4.Turning on the heater just removes the excess heat from the engine and places that heat inside the cab of the vehicle, but since you have hot air coming out of the vents, I would assume you have some coolant inside the radiator卋ut maybe not enough due to a leak, or not enough coolant circulation due to a restriction. A fast lube place can do the pressure test, and I would have them listen to your hissing noise as well?.before that becomes a bigger problem. Don抰 put this off, and don抰 drive this vehicle when overheating...this motor is expensive to replace! This hissing noise is probably due to steam escaping from a hole in the cooling system.
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