I RECENTLY REPLACED THE THERMOSTAT IN A 1996 FORD EXPLORER 4.0 4WD. THE HEAT WAS WORKING FINE AND THE GAUGE STARTED MOVING UP TO HOT AND THEN BACK DOWN TO THE COOL MARK WHILE DRIVING. AFTER COMPLETION I FILLED THE RADIATOR WITH WATER AND ANTIFREEZE AND THEN THE CAR'S HEAT WAS WEAK AND THE TEMP GAUGE WILL NOT GO UP. I LET THE ENGINE RUN AND TRIED TO BURP THE AIR OUT, BUT STILL NO GAUGE MOVEMENT. COULD IT BE THE TEMP SENDING UNIT OR THE GAUGE ITSELF? DID I NOT BURP THE AIR PROPERLY? WHAT COULD IT BE, IT'S GETTING TO BE COLD OUT SOON.

Could be that the thermostat has packed up...

Check all your coolant levels are correct and then take her for a good run out....keep your eyes on the temperature gauge,,if your temp gauge hardly moves then your thermostat is at fault....replace it.

If temp gauge works fine, you may be airlocked in your heater matrix... Try setting heater to full heat with fan on full power,, whilst car is ticking over, remove radiator / expansion bottle cap carefully and start squeezing your coolant pipes. This should force any air out through your rad/bottle top..

Please be aware that as your temperature rises in the engine. Your coolant will expand and start to flow out of the rad/expansion bottle..at this stage quickly replace the cap....check your heater...leave engine to cool....unscrew rad/bottle cap, check coolant levels, top up if necessary and squeeze pipes again..start car with heater/fan on full and repeat above.....

Hope this helps..

How do replace thermostat in 1999 Ford Explorer SUV YouTube

Hopefully this link may help.
Possible blend door not working 2005 Explorer Ford Truck Enthusiasts Forums

Bob

change the radiator fluids, and change out the thermostat, make sure the fan is working correctly

Is the engine heating up on the temperature gauge?

You probably have air in the system. Take off the radiator cap and start the engine. Turn the heater on hot. Let it warm up and fill the radiator as the level drops. Make sure the thermostat opens and coolant is flowing in the radiator. Continue to add coolant until the level stops dropping. Also check for any bleed ports on the hoses or housings where they attach. If you see any bleed ports, crack them open and let any air out. Put the cap back on and see if the heater is working. If this doesn't work, please get back to me.

could be a problem with the blend door. common on explorers. open glove box all the way down (bend in tabs to where it will open further than normal) . take flashlight and look behind it to top of black box inside dash. should be able to see tab for door moving while changing the temp. on the control unit. if it is broken should be able to hear cracking or popping from inside heater case.

head gasket has blown.

replace temp sensor and thermostat.
sounds like your engine has been over heating while the gauge was not working.

feel the hoses when engine is hot to see if the water is circulating,before you do anything else.If one is hot and the other cool then its not circulating,and heater core need a good flushing with some some caustic soda