No heat when stationary, heat when moving.

My 97 Sable has an interesting problem. When the car is running but stationary, like in stop and go traffic, the water temp gauge is showing rising temps but if I turn on the defroster there is no heat just cool blowing air. I only have heat when the car is moving, slow or fast it does not matter, and the temp gauge returns to normal. The heater core was replaced in July '03 by the local Mercury dealership and when the repair tech turned on the car to test it out he allowed the car to over heat resulting in a blown head gasket so the problem even exited even then. Since I did not use the heating system till it got cold I've only seen the problem now. I have taken the car back to the dealership and they saw rust in the coolent bottle, they claim that they did not see rust when the heater core was replaced. The dealership recommends that they water pump and other unspecified parts be replaced and the engine coolent system be flushed with an acid treatment - for a heafty service charge of course. I'm hoping to find out if they are taking me for a ride or are they telling the truth.

You didn't mention the thermostat, that's my first choice.Second choice is the water pump.If either one fails the car can overheat.When you're driving there may be enough airflow to cool the engine enough to lower the temp gage and when you stop there's no air flowing around the engine.You can check the thermostat operation visually.With the car cold, like first thing in the morning, pop open the radiator cap.Top off the coolant if needed so you can see it.Start the car.Wait a few minutes for the temp to rise.CAREFULLY peek into the radiator.When the thermostat opens you will see the coolant being pumped around inside the radiator.BE CAREFUL! The coolant can rise up and out of the radiator.If after a while you do not see the water moving and the engine is at operating temp turn the car off.Replace radiator cap.Feel the radiator hoses.If they are cold it could mean that your thermostat is not opening.This is the cheaper item to replace, I'd do this one first.If a new stat doesn't fix it then I'd look at the water pump.

Since making my post I have taken the care to a local repair shop. They found that the radiator was full of rust - they told me that Ford's raiators are know for rusting out, and the water pump was shot - the vanes were eaten away to the point were water would only be circulated when the pum was moving at high speed. After having those two parts replaced and a chemical flush the car is performing wonderfully, more heat then I know what to do with.

Glad you got her warmed up, I'll keep this in mind if mine starts fade.