BMW M3 Fuel System Problems: Starting and Idling Issues

Hey there! I had a similar issue with my 2005 BMW M5 last year (I'd say I have some experience working on BMWs, mainly basic maintenance stuff). The symptoms you're describing sound exactly like what I dealt with. In my case, it turned out to be a loose connection in the fuel pump wiring harness. The connector was corroded and wasn't making proper contact. My mechanic found it during diagnostics, cleaned the connections, and secured them properly. The whole thing cost me 95 Euro and fixed the problem completely. Before you spend money on replacing the entire fuel pump, definitely have your mechanic check the wiring connections first. I've seen this issue on several BMW models, including the M3. Could you share your current mileage and whether you've had any previous fuel system work done? This would help determine if we're dealing with the same problem.

Hey, just saw your message. My M3 has 50686 KM on the clock and last service was about 2 months ago. Funny you mention the wiring harness, I actually had my mechanic check it today. Turns out there's nothing wrong with the connections, they're all clean and tight. Getting a fuel pressure test done tomorrow to be sure, but the mechanic's leaning toward a faulty fuel pump. Really hoping it's something simpler, but these symptoms aren't looking good. Did your diagnosis include a fuel pressure test before finding the loose connection?

Hey again! Thanks for the update on your M3. Yeah, when I had my M5 checked, they actually did run a fuel pressure test first, it's pretty much standard procedure for these symptoms. In my case, the test showed inconsistent pressure readings, which initially pointed toward a bad pump, but my mechanic was thorough enough to trace it back to those corroded connections. Since you've already ruled out the wiring harness (bummer, as that would've been the cheaper fix), and you're getting the pressure test done, you're definitely on the right track. Just to share my experience further, when my connections were loose, the pressure readings were all over the place, jumping between normal and low. If your test shows consistently low pressure, then yeah, hate to say it but it's probably the pump itself. One thing though, have you noticed any weird noises from the back of the car when you turn the key to position II (before actually starting)? With my fuel pump issues, I could hear the pump making this weird buzzing sound instead of its normal whirring. Might be worth paying attention to that while you're troubleshooting. Let us know what the pressure test shows. Got my fingers crossed it's something minor, but if it is the pump, at least you'll know for sure and can get it sorted properly.

Thanks for all that detailed info! I actually tried listening for that noise you mentioned, turned the key to position II and yeah, there's definitely a strange buzzing sound coming from the back. Not the usual whirring I'm used to hearing. everything's pointing to the fuel pump, isn't it? Just got off the phone with the mechanic about the pressure test results, and surprise surprise, consistently low pressure readings across the board. Looks like I'll be shelling out for a new fuel pump after all. Really appreciate you taking the time to share your experience, even though mine turned out to be a different issue. At least now I know we covered all the bases before jumping into the more expensive repair. Guess I'll be booking it in next week to get the pump replaced. Thanks again for the help!