Ford Repair: 98 mercury sable, mercury sable, gauge needle

Question
hello Mark,i have a 98 mercury sable 3.0 automatic 6 cyl. sedan,with 64,000 miles on it ,,,,i cannot get any heat ,i have replaced the thermostat,,and the electric fan sensor,,,the temperature gauge needle is in middle when warmed up,,,i turn the heat knob to the red position,,then i turn the heat control, to blow heat out of the vents and floor vents ,i just get luke warm air and the air conditioner kicks on and off, and also the electric fan on the radiator comes on,,,,if i choose to blow the air on the floor ,the ac stays off  and the electrice fan runs on the radiator,and there is still no heat ,,,,i can feel the heater core lines ,1 is  very hot ,the other is not,,,also i notice if i warm up the car so that the temp gauge needle is in the middle of the gauge ,and then shut it off for only 3 minutes ,then start it up and put the heat on,, the needle drops down to almost the cold position,,,,,,,then will eventually heat back up ,,,,is there any control to allow the anti freeze to flow thru the heater core,,,or do i have other problems with the dials ,ac etc...thank you ,Joe

Answer
is your coolant brown these year models had a problem of coolant getting dirty not because of lack of maintenance but metal incompatability between freeze plugs and block if  yours is still clean your ok-coolant flows thru heater core at all times there is not a water control valve like older vehicles-your controls are working correct when you have it on floor the a/c compressor does not run but when on vents/floor it does to dry air in car. your heater core or tubes are probably clogged-try taking hoses off of core and connecting 3-4 foot of hose to each port on heater core then a water hose and flushing water back and forth if it flows thru freely move on to the metal tubes on engine the heater and bypass hoses connect to could also be clogged and is a lot harder to unclog i usually take them off and tap on the ground or with a small hammer until clog breaks loose or just replace them. as far as temp going to cold quickly make sure coolant is clean and full. good luck.