Heat not working unless engine at higher rpms.
My car started this morning with the low coolant light on. When I went to add more coolant, I pressure cap was loose. I tightened it and light went off. I added about a quart of coolant. My heater is blowing cool air until I accelerate from a stop or am doing about 50 mph. Then I blow nice and hot like I am use too. Is this a thermostat problem? What else could it be?