Explorer 4.0l 4x4 temp gauge goes to cold when heater turned on

check or change your terrmostat

change the heater/glow plugs or Spark plugs

Although the Explorer I had didn't have the same size engine, I did have a similar problem. Start with looking up the location of the temperature sensor unit. I little Google search turned up a few youtube videos and some links to new parts ranging from around $20 and up. Of course the price is only dependent upon the application so the lower cost one's could be for the smaller displacement models. Judging by your description and the fact that you drove that distance without the engine seizing, this may be the culprit. Good luck.

Best case scenario is your thermostat is stuck open. When this happens the engine will take a long time to warm up and will never get up to full temp. Worst case could be a serious problem such as a blown head gasket. If there is no coolent in the engine the temp gauge will not register either.

please state what engine ,diesel endura /petrol or a american big engine as in the europeon scorpio

check coolant level, if ok change thermostat, it may be sticking.

jtg

I just had a similar problem with my 99 exploder and the alternator was going out. Maybe have it checked.

Sounds like either a blend door actuator or temp. door actuator.

I think you could be right about there being air trapped in the system.
Or the heater core(radiator) is plugged.
Either way you need a pressurized radiator coolant pump to put the coolant in without air being trapped.
Although,i have seen it done by jacking the vehicle up high enough that the radiator was now higher than the heater core level, and then the coolant was put in.
But this is dangerous to try.