One problem related to power train has been reported for the 2003 BMW Z4. The most recently reported issues are listed below.
When steering sharply or turning the wheel frequently like to parallel park for example, the electrical suddenly completely shuts down in my car. The car's power completely dies and I lose ability to steer. Sometimes this is while making a quick turn with oncoming traffic approaching quickly. I must stop turning, wait for the power to turn back on, and resume driving as soon as possible. This is very dangerous and I am scared to drive my car.