I have a 1999 Sable and have no heat. We bought the car in the summer time and could not tell if the heat was working properly. The first step I took was making sure everything under the dash worked correctly. And yes everything under the dash works as it should. I then went on to flush the heater core. I flushed gallons and gallons through it until it ran clear water for a long time. That night we drove for about 25 minutes and once the car came to temperature, the heat came on so strong for about 10 minutes. So strong where it was burning our eyes and had to back it down. After that tenth minute, it turned stone cold and the heat never returned again. My next fix was replacing the thermostat. Following that no luck so I moved to the water pump. Both of which were extremely rusty. The water pump had all of the impellers intact. With the water pump I replaced both upper and lower hoses, and blew out the bypass hose along with running water through the engine block to check for blockages. Went for a road test and there was no heat and after a weeks worth of driving still no heat. I checked for coolant in the reservoir to see if any air pockets were trapped and the level dropped dramatically. I added coolant and no luck. I noticed tonight as I was driving in very cold temperatures, when you try to turn the heat on, the cars temperature drops to nothing and will return to operating temperature. This will happen every time you try to use the heat.

Thanks

 

Went out today and pinched off the bypass hose and running the motor around 3,500 rpm, the heat would work perfectly. Anything less and cold air would blow out.

 

What? Are you saying with a warm engine, the heater blows cold air when the engine has low rpm?

The cabin heating system should be independent of the engine speed. The coolant pump (called water pump) of the engine cycles plenty of coolant, and part of it goes through the heater core, which is like a little radiator. Sure, the faster the engine goes, the more the pump pumps, but the difference shouldn't be enough to affect the functioning of the heater core. It will get hot even when the engine is idling.

Does liquid flow easily through the heater core? If not, replace it.

In some vehicles this is a lengthy job because you need to remove the dash. I'm not sure what it takes in your car.