OK, so if I set Complex Engine Managment in the settings on what do I have to do to cool my engine and also, can someone explain me what's mixture and why to change it?
When complex engine management is selected, you have to adjust propeller pitch, mixture and radiator (cowl flaps) settings in order to get the best performance out of the engine and to avoid overheating.
Opening the radiator or cowl flaps allows more air to flow over the engine which increases cooling, but at the cost of a little speed because of the increased drag of an open radiator or cowl flaps.
Propeller pitch changes the amount of air the propeller 'bites' into. 100%pp is usually used for take off and climb. It's sort of like first gear on a car. You will generally get the best acceleration at 100%pp. In real life you usually lower the prop pitch to the 90% range when climbing to reduce engine temperatures and wear but you can leave it at 100% in the game. The higher the prop pitch, the more heat is generated so you will want to reduce it to the 80% range when you are in cruise. Each airplane in the game is a bit different so these are not hard and fast numbers, you'll have to experiment with each.
The higher the altitude the thinner the air so the mixture has to be adjusted for smoother engine operation. You don't have to adjust it too much in game below 10,000 feet. Increasing the mixture to 120% will increase the fuel component of the fuel/air mixture entering the engine and increase cooling but this will only work at very low altitude.
Power settings also have a large effect on engine temperature. Running around at 100+% power all the time will overheat the engine. The extra boost provided by WEP, water-methanol, etc will make you overheat faster.