No Heat on my '99

It's mighty cold in Michigan right now.

My 99 Taurus with 115K miles is running great. The only problem is that I've got intermittent heat.

Here's the background. A month ago I had no heat at all. The temp gage didn't move much either. After reading several other threads in this forum, I changed the thermostat and flushed the heater core. Now the temp gage goes just slightly above "C" but it takes about 10-15 minutes to get there. When I turn on the heat I get good heat for about 5 seconds. The air then gradually gets cold.

Is it the pump? Bad thermostat?

Could be a bad water pump. As they get old, the vanes wear out.
Bad thermostat-opening at the wrong temp. To test, put thermostat in a pot of water and heat on the stove. Measure the temp of the water when the thermostat opens (use a candy thermometer).
Restricted flow in the heater core- feel both hoses going to the heater core and see if there is a difference in temperature.
The fan-is it running when you first start your car. Was the fan wired to run constantly while the motor is running?
Maybe it is just to cold- some people put sheet metal, canvas (you may have noticed this on tractor-trailers), or cardboard in front of their radiators partially blocking air flow through the radiator.

It's mighty cold in Michigan right now.

My 99 Taurus with 115K miles is running great. The only problem is that I've got intermittent heat.

Here's the background. A month ago I had no heat at all. The temp gage didn't move much either. After reading several other threads in this forum, I changed the thermostat and flushed the heater core. Now the temp gage goes just slightly above "C" but it takes about 10-15 minutes to get there. When I turn on the heat I get good heat for about 5 seconds. The air then gradually gets cold.

Is it the pump? Bad thermostat?

If your temp guage registers only a slight movement beyond C it appears that the engine is not reaching the proper operating temp. This is usually caused by an open thermostat.

A bad pump would most likely cause a high temp because of reduced flow from the engine water jacket to the radiator.

First check that the antifreeze is at the proper level.

Check the temp sensor is working properly (a Haynes manual should tell you the specifics of the resistance values for a given temp).

Ensure that the new thermostat was replaced properly using a new gasket and mounted in the proper direction. If put in backwards you could get a bad seal and the thermostat could never fully shut the coolant flow to the radiator. If you feel that the unit is installed correctly, then you can either buy another one since they are cheap, or as KimMG suggests, test out the one you have.

Thanks for your feedback. I'm confident that my problem is not related to the thermostat or temp sensors. All of the coolant lines are hot once the car warms up. The coolant lines are only lukewarm at best going into and out of the heater core.

When I turn the heat on I get heat for about 5 seconds then it cools off. If I turn the heat off for a few minutes I'll get heat again for 5 seconds when I turn it back on. If I run the blower on the lowest setting I can keep the heat running for a longer duration.

Clear water runs through my core when I flush it. Could it still be a core problem? Is it possible that the pump has deteriorated to the point where it can't pump coolant through the core?

The coolant lines are only lukewarm at best going into and out of the heater core.

Clear water runs through my core when I flush it. Could it still be a core problem? Is it possible that the pump has deteriorated to the point where it can't pump coolant through the core?

If your engine is not overheating then I would think your pump is working OK. If the heater hoses are lukewarm then you could still have a partially clogged heater core. When you flush the heater core are you eliminating the bypass hose by connecting directly to the core inlet/outlet? Is it possible that the bypass hose assembly has a clog/restriction that passes coolant throught the bypass hose but resticts the flow into/out of the heater core?

Does it blow cold air when you run your AC pointing down to the floor? If so, try replacing your AC Blend Door Actuator.
At the same time make sure you don't have any vacuum leaks. Vacuum controls the blend door actuator. If this is an original AC Blend door actuator check some TSBs.