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Sounds like you may have combustion gasses creating an air lock in the cooling system. Or plugged heater core / radiator. Definitely a circulation problem of some kind either flow of hot antifreeze is restricted or the heater should blow very hot air especially when the car heats up so long as all the ducting and heater controls are functioning properly. If the vehicle has been overheated you more than likely have combustion gasses in the cooling system. Water pump will not work properly for combustion gasses that mix with coolant. Check to see if the coolant is beginning to get a little oily and dirty, this will indicate combustion gasses entering cooling system.
take the top off the filler cap run engine to boil. keep it full off water to the top. you will see air bubbles coming out as the engine gets hotter. squeeze the pipes on the rads to help the air come out. when fan kicks in turn engine off and leave to cool down.while you are doing this make sure the fan for heater is off but is on full heat. can have fan on number one on speed that is fine.
i would have the cooling system flushed and have the heater core flushed seperate half the time the heater core gets clogged with crud and you get very little coolant flow through causing no or poor heat.
You likely have a restriction in the heater coolant hose circuit - perhaps a coolant control valve not fully opening, or a kinked hose.
Insufficient coolant volume is flowing to keep the heater core hot.
Try back flushing the heater core system with a garden hose to check flow.
Did you run engine from cold, with heater on when you refilled the system? Try again, rad cap off, with heater on, and let engine get up to operating temperature, make sure you have good flow through rad ,leave cap off, until rad fan comes on to cool returning coolant to rad at least twice. Give engine a little extra rev, for 5 seconds or so, every once in a while to make sure flow getting through heater core, and add coolant as necessary. See if that helps, as it may just be an airlock in system. If your cooling system has any bleed screws incorporated, crack them once in a while as well.
Just finished fixing my daughters 1999 taurus heat. It would blow warm only when running down the road. At idle it blew cold. I started with replacing thermostat, and water pump because they are the easier fix. Still did not help. Then I replaced the heater core. A daunting job but not that hard to move the dash slighly out of the way to replace. There is a good write up at tarusclub.com. After that, heat was better but at idle, it would still blow cool sometimes. Researched and found answer. Park car on the steepest hill you can find with front of car up. Start car and take off reservoir cap. Let car warm up and keep rpms about 1500. After a while the fluid will come running out of the car and all over the place but it will bring air bubbles with it. Reduce rpm, add more fluid and repeat about 3 times. It will make a mess but after that, the heater works perfectly. We get heat long before the temp gage indicates any and it never blows cold unless you lower the temp.