I have a 2000 Ford Taurus sedan SE with around 125,000 miles. First of all the heater works 95% of the time but when it warms up and it hasn't been that cold here it takes awhile to warm up while idling. It seems like when I get the engine going and get on the highway the heat cranks up and is very hot like nothing is wrong. It just seems like when it is at a stop and idling that the heat "sometimes" gets cold again. If it is the thermostat where is the thermostat located on this vehicle? I have changed many but I just need to know where it is. The waterpump was replaced this summer also and the radiator doesn't seem to have anything clogging it. Thanks in advance for the help. Take Care.

I don't think a thermostat would cause the heat to start blowing cool after the car is up to temp. What does the temp gauge do as the heat starts blowing cooler?

I suspect either you have an air pocket in the system or maybe the water pump impeller on the new water pump is backward.

Why was the water pump replaced last summer?

-Rod