BimmerFan52

If you are running straight water in your cooling system in the belief that it will run cooler than running the proper ratio of distilled water and coolant then you are mistaken. Straight water will not run any cooler.

The proper BMW coolant has corrosion fighting ingredients that will protect the engine and other cooling system components and is critical to engine longevity. And coolant acts as a lubricant (rub some coolant between your thumb and forefinger to feel it) and will greatly extend the life of your water pump.

I strongly advise you to immediately change back to the proper ratio of BMW coolant and distilled water.

I am seeing the same coolant temperatures in Scottsdale AZ. The N62B44 engine in the 545i is designed to run at higher temperatures to be more efficient in burning fuel. I don't see great variations in coolant temperatures based upon ambient temperatures, but rather how hard the engine is pushed.

I also advise you stay with an oil such as Mobil 1 0W-40 that can hold up better to higher temperatures without shearing down quickly.