1998 ford expedition: engine ovr heat light comes on- soon after heater goes from hot air to cold. Pull over cycle key all lights go to normal soon after heater goes to working. replaced radiator- fan clutch-thremo stat no help.

Hi try this video it may help you find the fault it covers the codes on 4.0l engine. Ford Explorer 4 0L SOHC Rough Idle P0171 P0174 Lean Codes

May a a vacuum or electrical problem that actuates the blend door.

No hot air is due to low coolant, or even poor coolant flow. The engine heats up the coolant, and it passed through your heater core, and the heater blower motor pushes air through the heater core to warm up the air. If your "fan"is working, but the air is cold, make sure your coolant level is good.

Bad heater core.

Try the heater controles.There is a possibilaty with this expedition that the heater doors are not opening properly.When motor is at low RPM the vacum the motor pulls is low to.This vacum is what controls the doors and vent.You can pull vacum lines on and off ONE at a time and listen for the doors to open.There is also a good possibilaty the vent door contol switch at the dash could be bad.You would need a vacum gauge to test it though.

I ASSUME THIS IS A 98, THIS PROBLEM IS CAUSED BY A PART CALLED AN "AIR TEMPERATURE CONTROL BLEND DOOR ACTUATOR MOTOR" THIS MOVES A DOOR BETWEEN THE COLD AIR FROM THE AC AND THE HOT AIR FROM THE HEATER CORE, YOU WILL NEED TO REPLACE THAT ACTUATOR. .

your evporator may have a leak or you heater core may be leaking that's the reason for the mist

An overheating engine will often shudder and make a unique popping/rattling noise. Is it doing that? If it ever does that it's time to call a tow truck or further risk major engine repairs.
Try turning on your heater. Your heater makes its heat by blowing air through a mini-radiator (like the one at the front of your car) and pushing that hot air through to the inside of the car. Is the air coming out of the vents warm to normally hot or I-just-opened-the-oven make your skin tingle hot? Check and make sure you have coolant (when the engine is cool) and fill to where it should be. There may not be enough to circulate through the cooling system but enough to hit the temp. sensor and give you warnings.

the climate control panel power goes through the ignition switch. So, it sounds like you could have a switch issue.
Do other things work witht he key off as well? Like the wipers, radio

Do you have electronic or manual heater/a/c

If fuse #21, 50 amp, and its yellow 37 wire to the ignition switch is somehow hot with the key off, it could be the problem.
Pull that fuse in the power box in the engine compartment & see if it shuts off.

Is the blower fan for the heater going on, but only cold air? If the fan is working, but no hot air situation, 1st check the coolant level, if it's low, the coolant won't flow thru the heater core, so little or no heat. If level is ok, what does the engine temp gauge indicate? Is the engine running normal temp range, cold, hot? Let me know answers to these questions so I can help u.