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I recently bought a nice one owner 78 f150 2 wheel drive with a 300 automatic. My problem is the heater works fine when the vehicle isn't in motion, but as soon as I take off the temperature guage drops and so does the temperature in the cab. I have replaced the thermostat with a 160 degree unit and replaced the water pump. Still nothing. I also noticed the fan shroud is missing, but I figured it would help me if I had an overheating problem. Any help?
Are the both heater hoses getting hot? It could be a plugged heater core. Also sounds like the thermostat is on the low side, you might want to try a 180* or 190*
190 for all year round. For low ambient temps use a "Ford Thermostat" with a 300-6. Put a piece of cardboard in front of the radiator that cuts off about 1/2 of the area.The cold air rushing over the sides of that engine will pull more heat out than you can generate. Or you can buy a canvas "winterfront" like the big rigs use.
Put the fan shroud on. The engine will overheat without it when it gets a little warmer.
Make sure all of your baffles are in place also. Believe it or not they help keep the engine warmer in winter and cooler in summer.
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