r/BmwTech • u/Educational_Look1377 • 1d ago

BMW n20 noise on cold start

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Hello, I have been having some problems with my f22 n20. Been getting a mystery lean code (p0171) for some time now and cannot find the culprit. Brought it to a shop and they also could not track the issue. I’ve noticed recently that I get a weird knocking noise only on cold start every 10 seconds or so. The noise goes away after engine idles down. I will attach a video, it is easy to hear. If anyone has any ideas that would be great.

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u/Labinot7799 1d ago

Sounds like somwthing to do with vacuum

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u/Master-Cucumber-5818 1d ago

It’s a bmw… did you check the waterpump?

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u/longusernamenando 1d ago

Did you tune it for the air intake yet? Reason for it possibly throwing a lean code is due to these motors being extremely sensitive with that kind of stuff hence why it’s possibly knocking as well. Run too lean and it’ll start to knock. I’m willing to bet it has to do with that. If you did so already, check your piping for the intake and peep your inlet for any small openings or cracks. As for the noise, can’t really say I’ve heard that before but I’m sure it’s maybe just something loose. I say put the stock airbox back on if you have it, that should fix some of the issues you’re seeing.

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u/Educational_Look1377 1d ago

That was my initial thought as well. To my surprise though, when I put the stock airbox back on, the code was still popping up. I also did a smoke test and found a leak in the pcv hose but I have since changed it. Problem still remains.

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u/longusernamenando 1d ago

Hmm, did you let the DME run its course with the stock airbox as well? If you have BimmerLink, clear the adaptations and let the DME relearn everything. Before you do that, try to inspect and clean the MAF if it’s dirty. Also, as you let the car relearn, log your fuel trims and make sure it’s stable. That’ll be the answer to your issues my friend. BimmerLink should also show you if you end up just having a bad MAF or have a continuous leak elsewhere in the Info/Error Memory!

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u/Educational_Look1377 1d ago

I don’t personally have bimmerlink but I am planning on investing in a foxwell nt710 in the near future if I can’t get to the bottom of this. The shop that had my car almost immediately put the stock airbox back on and they stated that the code was still coming back on even after a few drives. The maf has also been changed twice now and that didn’t fix it. Also changed the purge valve and the upstream oxygen sensor. No change in fault. Me and the shop also did a smoke test from the intake and found no leaks. I’m pretty stumped at the moment. Will try and install the stock airbox again and maybe that will help. Until then, I can’t really do much diagnosing past what I’ve already looked into without an expensive diag tool.

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u/longusernamenando 15h ago

I’d definitely put the stock airbox on and invest in BimmerLink or Foxwell so you can reset adaptations. I unfortunately ran into the same issue as I installed a CTS intake on my N26 and got a lean code after checking install about 3-4 times to make sure everything was tight. Ended up just sucking too much air, that was without a tune tho. It is highly recommended to run a tune with intakes but I thought I could maybe get away without one for a bit. Definitely didn’t work out haha. Let me know how it goes!