Your car needs a few things to cool the engine.
1) Volume. Do you have enough water to circulate through the system? Do you have any leaks that are preventing the system from holding enough water?
2) Movement. Obviously your water pump moves water through the system in a circle. Is your pump being turned by the belt? Are their missing impeller blades? Is the thermostat opening so water can move? Is the radiator clogged with contaminants from using tap water or the wrong coolant? (non-Genuine BMW)
3) Efficiency. Your cooling system is less efficient when cavitation (Bubbles) start forming. This happens when water boils. Water boils when it reaches boiling temperature. Two things increase the boiling point. Coolant and pressure. Make sure you are half-filled with coolant and that your cap is of the right spec and functional.
4) Heat transfer. This happens when all the above demands are satisfied and air is moving over the fins of the radiator. When your car is at speed, natural airflow accomplishes this. While stationary, your fan, whether it be mechanical or electrical accomplishes this. Is your fan in check? Is the aux. fan switch working turning on the electric puller fan?
In the end, you should replace your entire cooling system. Minus 5 Mango points for not reading my sticky.
Last edited by Mango; 05-17-2012 at 08:58 PM.