First of all, i have an S5 NA with an S4 engine(IT'S A LONG STORY!!), only using the S4 "block". The car runs great, no problem at all. Have a new Godspeed radiator, new electric fan. My gauge temp. it's always at 50% normal temp. don't know why. After some miles running out the car, the gauge temperature starts go up. First like 15% up. If i still run the car after that it get 25% up(way too much). I don't know why is overheating. Have no leaks, new radiator, new electric fan. What i've searched is that the temperature sensor of an S4 gives different temperature than an S5 temp. if i'm using an S5 gauge. Any ideas what might be?? I'm from Monterrey Mexico and the normal temperature here is like 113 F so i really don't know if that makes a difference. Greetings.

 

air in system, search for bleeding the coolant

 

The temp sensors in S4s are different but that wouldn't cause the car to over heat like you are describing. S4 temp gauges go up about a 30% of the way at normal operating temperature, s5 gauges go up about 45% of the way at normal operating temperature.

 

get rid of the electric fan and run stock fan and shroud, 113 degrees is brutal heat and no electric fan set up will ever cool your car as well as the mazda set up.

make sure you have a undertray as well, OEM thermostat, make sure your water pump is actually pumping water, if its of questionable age replace it.