I have a 99 V6 4.0 Ranger and a couple days ago my Temp Guage started acting up. I would start my truck and it would warm up to normal (Mid temp) and then my heater in the car would go cold, then I would see my engine temp gauge go down to cold, then go almost to the red, then back to Normal. After this the heat would go back on.and everything would be fine for the rest of the ride. I have just replaced my thermostat and this morning it did it again. This happens everytime I first turn on my engine, from sitting for a while.

Any suggestions on what is wrong?

 

maybe you have air in your system? open the rad cap and squeeze the top rad hose to try and purge some air out

 

I did see that in a couple other issues kind of like mine. I will try that and see if that helps. Could it also be a bad Cap?

 

i guess if the cap isn't working right it would let air in, im not sure tho