2007 nitro, heat goes away while driving. heater core is not blocked. water pump and thermostat have been replaced. blend air door is functioning properly, no sign of leaks.

There are several possible reasons why you may be experiencing cool or no heat in your 2002 Jeep Grand Cherokee Laredo after replacing the thermostat and bleeding the air out of the lines. Some possible causes could include:

1. A malfunctioning blend door or actuator: The blend door controls the mix of hot and cold air that is directed through the heating system. If the blend door or actuator is not functioning properly, it may cause a lack of heat.
2. A clogged heater core: The heater core is responsible for transferring heat from the engine to the cabin of the vehicle. If the heater core is clogged, it may not be able to transfer heat effectively, resulting in cool or no heat.
3. A faulty thermostat: If the thermostat was not properly installed or is not functioning correctly, it may cause the heating system to not work properly.
4. A problem with the water pump: The water pump is responsible for circulating coolant through the engine and heating system. If the water pump is not functioning properly, it may cause a lack of heat.

heater hoses run from the rear of the block to the water pump. One hose into the heater should be hotter than the other because heat is transferred in the heater in the car. . The cooling system draws water from the bottom of the radiator ( water pump inlet)which is always cooler than the top hose and dispenses it throughout the engine and heads including the heater core. It is then pushed past the thermostat with all the heat from the engine to the top of the radiator where it is cooled down by the air passing through the radiator . Check that the air vent for the cabin is set to recirculated cabin air and not fresh air from the outside as it will require recirculated air to increase in temperature to work properly

Good point. Solving it will probably take more time than money.
Thermostats and heater cores do not work one day and stop the next. If your engine warms up normally and doesn't overheat, the gauge reads normally, engine is at normal operating temperature after about 10-15 minutes ...all these point to a good thermostat. It is working right and should not be a problem.
To check the heater core, wait till engine is at normal operating temperature. Turn heat setting to high, then feel both heater hoses-both should be hot. If both are hot the core is circulating coolant and is not a problem. If only one is hot (which would be the inlet hose) the core is not circulating-could be an air block or the core is plugged up and will need replacing.
If heater core and thermostat are good, then it probably is the temperature blend door in the heater case. The door actuator is probably vacuum controlled. I don't know how difficult it is to troubleshoot on your Taurus, but check if vacuum controls are working right from the dash controls, check if the blend door is stuck, is broken, or if the vacuum control is working to open and close the blend door. When you want heat, the blend door should let blower air pass across the heater core.
From the engine you will have a single vacuum hose that goes through the firewall into the dash to the heater/AC controls. There, a pod like device can rout the vacuum signal to various doors on the heater case.
Good luck.

Check for air in the cooling system - tighten carefully all hoses especially ones by expansion tank. If motor on cool down sucks in air instead of water an air bubble could restrict the flow of coolant into the heater core.

Look for an air block in the heater core, or a water valve in the heater hoses that may be closed. The normal way to purge air from the coolant system is to run the engine with heater on high heat, and with the radiator cap off, until the thermostat opens and the engine is at normal operating temperature. Then shut off engine, top up the radiator and overflow tank, and replace the radiator cap. If hot coolant is still not circulating through the heater core, you may have to get inventive and crack the outlet line loose to get flow started.

Are the heater core hoses getting hot? That could be a blockage in the intake manifold or engine block. Is the blend door sticking? That would be one item that would make for lukewarm air if the hoses feel nice and hot when up to operating temperature.

LOOKING AT HEATER CORE TOP HOSE COOLANT FLOW IN BACK OUT THROUGH THE BOTTOM HEATER CORE HOSE CHECK WATER PUMP BYPASS HOSE SEE IF IT GETS HOT.IF NOT COULD BE BLOCKAGE AT BYPASS HOSE FROM WATER PUMP OR WATER COULD BE FAULTY GETTING TOO WEAK PUMP COOLANT THROUGH SYSTEM.IF COOLANT GETTING HOT GOING TO HEATER CORE THE HEATER CORE AND COOLANT SYSTEM NEED FLUSHING OUT.

I would 99% doubt its a clogged core. Have the blend door checked (its a door that cycles between hot and cold)

If you have good heat on one side, it can't possibly be the the thermostat. Good heat on one side means that there is plenty of hot water (water and antifreeze mix) circulating through your heater core. The thermostat controls the temperature of the water. If you have hot water, the thermostat is not at fault. Your Impala is equipped with what is called "zoned climate control". All the inside air (both sides) is still heated by the same heater core. The difference between a "zoned" system and a conventional system is that there is more than one "blend-air" door insided your HVAC housing. The blend-air door swings back and forth to direct the incoming air either through the heater core (for hot air) or around the heater core (for cold air) The drivers blend-air door may be broken or the motor that moves the blend-air door is malfunctioning.

Where the rubber hose goes into the heater, there should be an air switch that opens an closes a shut off to the heater coil try it manual it should open an close by hand