I have a new 4.0 engine with new radiator. changed out the thermostat twice and the coolant sensors twice..
Problem is, engine temp gauge keeps moving from cold to past the mid point and back to cold. Repeatedly. Help please.

 

There is a good possibility of a poor connection in the wiring from the coolant sensor to the temp gauge. Some temp gauges contained the voltage regulator for the other gauges also. If the wiring checks out OK the gauge itself is malfunctioning.

 

Thanks Hanky, I wiggled the wire and the gauge started working. Your a genius. Thanks

 

Sometimes we get lucky !