1994 Nissan Altima 2.4L automatic transaxle
In cold weather (around freezing) the car warms up fine and blows warm air if I am stopped or driving at slower speeds but as soon as I get on the interstate and drive 60 to 65 mph the gauge begins to drop until it is under the C for cold and the heater fan blows cold air. After I get off the interstate and drive at surface road speeds and stop and go the engine warms back up.
I replaced the thermostat and coolant but no change. What could be causing this and what is the fix?
Certainly symptoms of a stuck open thermostat.
Re-check the thermostat for proper installation (misaligned or upside-down), or even defective and stuck open or opening early. Other than that, be sure all air is bled from the coolant system.