My 2001 525i wouldn't start. Just a "clicking" sound when I turned the key. Thought I may have had a dead battery; friend provided a jump. Still just a "clicking"--would not crank. Disconnected jumper cables, THEN I discovered I did not replace my gas cap the night before after filling with gas. I replaced the gas cap. With the jumper cables NOT connected, turned the key again, Bingo! Started right up. That was three days ago. No problems since then. Was the problem the gas cap or do I have a problem with my ignition system? If it is the gas cap, why did it start the night before when I left the gas station with cap "off"?