taurus with no heat on 1999 Ford Taurus

I have a 99 taurus with heat issues. When the car is sitting still for a long period of time the heat will work prety well. As soon as the car begins to move, the temp guage nose dives and the heat blows ice cold. The thermostat has been changed, blend door is working correctly, heater core is working just fine, fresh coolant at the correct level also. We are kind of stumped at this point. Any ideas?

Asked by for the 1999 Ford Taurus
2 answers 3 comments
Sounds like the cooling system is working too well. Since the radiator is where the heat is removed from the coolant, it seems as though the radiator is working too well. This could be from the fan staying on all the time, or the coolant is moving through the system too fast, thus being too efficient. You can try a thermostat that allows the engine to run hotter, and make sure the fan is not coming on too soon or staying on too long. The coolant should also be mixed properly, 50% water, 50% coolant.
Good Luck, and thanks for providing excellent info about the problem your experiencing!
Unfortunately, the fan isn't staying on. It only kicks on after the car has been running while its sitting still for a while and the temp gets to the middle point, just as it should. Another stumper! And the coolant is 50/50. The only other thing i can figure is the water pump. I've heard the impeller on them corrodes after time and breaks up into the cooling system, therefore the coolant isn't able to circulate through the system. Explains all the clogged heater core issues these things have too. Its my next step. Wish me luck.
You may be right on about the impeller. If it was not working at all, it would overheat, but if it was pumping at 50% it may run cooler. How does the cooling system look inside, is there any accumulation of slime or gunk? Have you tried using a different temperature rated thermostat?
Just flushed the system out. Pretty nasty..
Just joined here so this might be a bit late but couldn't the problem be from low coolant. I remember my mechanic telling me that the car will run cool if the coolant is low? Thus the reason the temp gauge drops when the engine is at higher rpms.