One of our cars is a 98 Ford Taurus. The heat doesn't work very well. Never had the heater core melt down with the collant leaking onto the cabin floor.
The engine doesn't get overheated. But warm air will not blow through the cabin vents no matter which setting is used. AC works just fine.
With the engine running, warm, and heat selected to hot on the cabin controls, the heater core hoses are warm, but not hot. You can squeeze them and they feel a little spongy.
Thermostat was replaced and put in the proper direction. No change.
Things I can figure that might be worng:
-The heater core is plugged or otherwise broke. $500 fix if it has to be replaced.
-There is a heat/cooling blend door on the 90's era Tauruses that is known to get fouled up.
-There is some valve between the water pump and heater core that is plugged up. But I get see where there would be one.
-Perhaps the water pump is not working so good? (But then why am I not overheating?)
When the system was flushed a few months ago, I got decent heat, but not fantastic. It eventually went away. That's why I'm thinking there's some valve the is not opening. But I'm not sure.