[Bajabrent]

Hi, i have a 99 Explorer XLT, that has a hyperactive temperature gauge. When car is idling it will shoot way past the red zone and remain there for a few minutes, then drop back to normal, for awhile and then repeat this spastically

Water/Radiator fluid level is fine, water is not hot. Does not appear to be related to thermostat. Figured I'd replace the sensor, so bought the part and had my Mexican backyard mechanic replace(I live in Baja). Seemed to work at first, but w/n an hour, repeat. Of course the "CHECK GAGE" light also turns on.

. :dumbass: = correctly depicts my comprehension of auto repairs. But I'm in Mexico and not easy to take this to a dealer.

 

[99SportX]

It could be a wiring issue somewhere. The wire right at the connector could be partially broken causing an intermittent connection but in that case it would read low. If the sensor is unplugged, the gauge should read C. If you ground out the connection it should go up to the H. The wire could have worn through somewhere and it is touching some bare metal on the engine or body.

One thing that can happen is that if there is air trapped in the system, you can get real high temps from steam up by the sensor. Make sure that the thermostat is installed such that the jiggle pin (if it has one) is located at the highest point. This would only be an issue on the 4.0 OHV V6 and 5.0 V8 engines. The 4.0 SOHC has the thermostat installed vertically.