One problem related to lose power steering has been reported for the 2004 BMW Z4. The most recently reported issues are listed below.
On warm days, vehicle steering is sticky, hard to turn. Feels like lose of power steering. Vehicle also pulls either right or left depending on last movement. Hard to steer back to center. Upon further research, problem has been found to be the steering wheel column, but no action or recall has been taken. It needs to be recalled.