Fan not comming on when temp gauge rises in my 2009 BMW X3

Check for bad temp sensor

Sounds like you have either a bad temperature sending unit or a shorted wire for the temp gauge.

is the cooling fan cutting in ? run the engine whilst stationary with the hood up , give it a little gas 2000revs and wait for the temp gauge to rise ,the cooling fan should be heard cutting in , and off again once cooled the engine temp down ,this may take 10-15 mins but you have to be sure the fan is cutting in or you may cook the engine

the way to check the thermostat is with the motor hot feel the upper radiator hose to see if can squeeze it. you should be able to squeeze it. the water pumps is big problem to

Loosen of the tension on the belt. see if there is any movement in the pulley (up-down -in -out Spin the pulley and check for roughness in the bearing.; Check for water leaking from the small hole in the housing at the base of the pump housing . Refit belt and correctly tension if nothing found. Check when it over heats (idle at lights) hard runs (speed) starts when cold and temp gauge continues to rise after engine is hot check if fan comes on at all.. Is the engine actually getting hot or is it an indication from the temp gauge ( faulty temp sender). If all is ok engines run hot if they are running lean so check the o2 sensor in the exhaust

What degree thermostat did you have put in it they make all different degree thermostats.

Try the overflow tank cap. The seal will go bad and not hold the psi causing the water to flow out this is something you will not always see which will cause the engine to overheat or temp gauge to rise. Also the thermostat could be sticking and cause the temp gauge to rise. Are the fans turning on when the temp gets high? If so then I would resort to the above first. If the fans are not coming on, the relay or fan/s could be the problem.

check the water level in the radiator and in the expasion tank,
I reckon the thermostat has failed (very very likely).
http://auto.howstuffworks.com/cooling-system8.htm

when it fails it never opens to circulate water to the radiator, if this is true your temp gauge will read HOT but the radiator will be quite cool compared to the engine. ------ Thermostat is around $60 / £48 from BMW only!!, a non-bmw part is false economy and will invalidate your warranty.

they are designed not to fluctuate with engine temp and are kept stable once operating temp is reached. only if the temp exceeds a set mark will the needle rise to indicate overheating,