I have a 96 that the temp gauge stays about 140 most of the time unless I leave the truck idilling then it starts to get hot, After driving it hard it will have coolant overfilling the overflow bottle and the truck will have never got above 190. The gauge is working because I got it hot once and the gauge went up. It also gets terrible milage and im wondering whats wrong and if the problems are related thanks for any help .

 

maybe a boost leak

 

Replace the radiator cap.

 

also do a radiator/cooling system pressure test