BMW Z3 ECU Troubles: Hard Starts and Power Loss

Hi there! I experienced almost identical issues with my 2005 BMW Z4 last year. The symptoms matched yours exactly, slow starts, power loss, and unusual system shutdowns while driving. Initially thought it was the ECU too, but turned out to be a failing main relay. Had it diagnosed at the garage and they found the relay had worn contacts causing intermittent electrical connections. This affected the fuel pump and various control systems. The fix involved replacing the main relay and cleaning the connection points. It was a straightforward repair, though accessing the relay location required some disassembly work. The problem hasn't returned since. While it wasn't a major repair, ignoring these symptoms could lead to getting stranded or more serious electrical system damage. I'd suggest having it checked soon at a BMW specialist who can properly diagnose relay vs ECU issues. Hope this helps!

Thanks for sharing your experience! That's really good to know about the main relay, sounds like it could be the same issue. Would you mind telling me roughly how much you ended up paying for the whole job? Also curious if you've had any other electrical gremlins pop up since the repair?

Hey again! Just wanted to follow up, glad my previous reply was helpful! Since I saw my mechanic about this, it turned out you and I definitely had the same issue. It was indeed the main relay causing all those headaches, though they also found my ECU needed attention during the diagnostic work. The total came to 120€ for replacing the main relay, and honestly, it was money well spent. Since getting it sorted, my Z4's been running like a dream, no more of those annoying electrical quirks or starting problems. The unusual system shutdowns are completely gone too. Best part is I haven't had a single electrical gremlin show up since the repair, which was about a year ago now. The car starts right up every time, and the power delivery is back to what it should be. The mechanic did a thorough job cleaning all the connection points while they were in there, which probably helped prevent any new issues from popping up. Trust me, getting this fixed sooner rather than later is definitely the way to go. Let me know how you get on with yours!

Thanks so much for the detailed follow-up! I actually took your advice and went to see a mechanic yesterday. You were spot on, it was indeed the main relay causing all the trouble. The mechanic showed me the worn contacts and explained how it was affecting everything. The repair cost me a bit more than yours since they found some corroded wiring that needed fixing too, but still way less than I feared for a BMW repair. The car's already running so much better, starts up immediately and the power is back to normal. No more unusual shutdowns either! I feel pretty relieved I didn't wait any longer to get it checked out. My old mechanic had me worried it might be a major ECU failure, which would've been way more expensive. Really glad I asked here and got your input. Sometimes the simplest solution is the right one! Thanks again for helping me avoid what could've been a much bigger headache. Now I can finally enjoy driving my Z3 again without worrying about getting stranded somewhere!