Quote Originally Posted by BlueM60
I cant imagine why it would be reccomended to have the engine as operating temperature. I think it should be cold, you dont want to hit a hot metal with cold water, this can cause cracks, deformation ect. No matter what type of metal. A cold engine with a good degreaser and mild pressure will do the trick.

-Ben
This is a myth.

Just imagine what happens when you start the car in sub freezing weather, jump on the freeway and the T-stat opens. The engine is suddenly flooded with super frigid water from the radiator. No cracks.

Your engine is designed to handle this stress.

I have seen old cast iron exhaust manifolds on inline 6 cylinder cars crack this way when they don't have good methods of allowing for expansion, but BMW manifolds are well designed.