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      07-31-2021, 12:50 PM   #1
TripleU
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Hi guys, recently my M2 has really rough cold starts where the engine starts off sluggish, and RPMs dip below 500 RPM where the car sounds like it is about to die before going back up.

This happens only during cold starts and the car starts up no problem after that during the day. Any info what is the problem?

Car is stock with just under 30,000 miles.
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      08-15-2021, 02:42 AM   #2
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Might be time for new spark plugs.
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      08-15-2021, 01:47 PM   #3
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spark plugs or waste gate being loose possibly
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      08-16-2021, 11:42 AM   #4
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With a fully warmed engine and the car in neutral, disconnect the MAF sensor and let it run a few minutes. Shut the car off and reconnect the MAF sensor. On the next cold start, the issue should likely be fixed.
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      10-24-2021, 11:35 PM   #5
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Quote:
Originally Posted by XutvJet View Post
With a fully warmed engine and the car in neutral, disconnect the MAF sensor and let it run a few minutes. Shut the car off and reconnect the MAF sensor. On the next cold start, the issue should likely be fixed.
Thank you so much! A few weeks ago, my car stalled trying to do a cold start so I tried your method and now my car starts up no problem now. *Knock on wood*
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      09-11-2022, 06:23 PM   #6
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Quote:
Originally Posted by TripleU View Post
Hi guys, recently my M2 has really rough cold starts where the engine starts off sluggish, and RPMs dip below 500 RPM where the car sounds like it is about to die before going back up.

This happens only during cold starts and the car starts up no problem after that during the day. Any info what is the problem?

Car is stock with just under 30,000 miles.
I have had this exact same behavior on my '16 M2 since I got it a year ago (did the same cold start stumble on stock tune and the carbahn tune). I tried the MAF sensor unplug procedure listed above last week, but nothing has changed. I'm curious if any of you M2 owners (or N55) just live with this? Or is this cold start stumble really worth chasing down multiple rabbit holes? It's amazing how many people have stories about what has seemingly fixed this, but no consensus yet on what it is the definitive solution? Thanks in advance..
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      09-15-2022, 10:20 PM   #7
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Quote:
Originally Posted by StateVector View Post
I have had this exact same behavior on my '16 M2 since I got it a year ago (did the same cold start stumble on stock tune and the carbahn tune). I tried the MAF sensor unplug procedure listed above last week, but nothing has changed. I'm curious if any of you M2 owners (or N55) just live with this? Or is this cold start stumble really worth chasing down multiple rabbit holes? It's amazing how many people have stories about what has seemingly fixed this, but no consensus yet on what it is the definitive solution? Thanks in advance..
Have you changed your spark plugs?
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      09-16-2022, 05:24 AM   #8
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I've seen the same thing on my car a number of times, interestingly it started last May/June, persisted through the summer then Fall/Winter it went away, came back this May/June. My spark plugs were changed last year as well.

I'd be curious if anyone else has a slight ticking noise from the fuel tank breather line and the two things are related.
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      09-16-2022, 08:55 PM   #9
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Quote:
Originally Posted by track.n.trail View Post
Have you changed your spark plugs?
Yes, I changed them out for new ones about 8 months ago and nothing has changed. I don't hear any ticking from the fuel tank breather line either..
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      09-17-2022, 12:41 AM   #10
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Quote:
Originally Posted by StateVector View Post
Quote:
Originally Posted by track.n.trail View Post
Have you changed your spark plugs?
Yes, I changed them out for new ones about 8 months ago and nothing has changed. I don't hear any ticking from the fuel tank breather line either..

Did you check for any error codes?
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      09-17-2022, 10:45 AM   #11
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Quote:
Originally Posted by KevM2 View Post
Did you check for any error codes?
No, that's probably a good idea. I have had the car in for service several times for routine maintenance at Winslow BMW and I assumed they would scan for codes, but maybe they don't unless the CEL is on.

I just keep reading about this issue on the M235i forums and other N55 applications where nobody seems to have errant codes, and some of them never get it "fixed". Some think it's an error in the cold start tables that just builds up over time or something...who knows.

I'm bringing my car in for an oil change to my independent BMW tech who has done good work for me in the past...I will have him chase this a little and see where it goes...
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      09-17-2022, 11:10 AM   #12
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Pretty sure all these recent idle complaints are due to injectors getting older/used and either getting clogged up or beginning to leak.

Mine is in the shop for something similar and two plugs were black. Ran perfectly fine under load and at WOT, just a bad idle that got worse and worse over the last year. Kind of indicates leaking injectors in those cylinders, leading to rich mixture misfires at idle.

Doing new injectors early this week & will let you all know.
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      09-17-2022, 11:16 AM   #13
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Quote:
Originally Posted by ZM2 View Post
Pretty sure all these recent idle complaints are due to injectors getting older/used and either getting clogged up or beginning to leak.

Mine is in the shop for something similar and two plugs were black. Ran perfectly fine under load and at WOT, just a bad idle that got worse and worse over the last year. Kind of indicates leaking injectors in those cylinders, leading to rich mixture misfires at idle.

Doing new injectors early this week & will let you all know.
Thanks. Was your "bad idle" all the time or just the first cold start of the day? My cold start idle stumble just happens on the first cold start of the day and then it's fine the rest of the time, even sitting for 8 hours. The idle while running in neutral while driving (at stoplights) is also normal. It's just that very first cold start that does the strange stumble-->dip-->rev up--> then normal idle from that point on. This happens like clockwork when the car sits for about 24 hours.
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      09-17-2022, 11:28 AM   #14
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Quote:
Originally Posted by StateVector View Post
Thanks. Was your "bad idle" all the time or just the first cold start of the day? My cold start idle stumble just happens on the first cold start of the day and then it's fine the rest of the time, even sitting for 8 hours. The idle while running in neutral while driving (at stoplights) is also normal. It's just that very first cold start that does the strange stumble-->dip-->rev up--> then normal idle from that point on. This happens like clockwork when the car sits for about 24 hours.
Mine acted just like that, and then got worse and worse and started doing it all the time.
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      09-17-2022, 01:48 PM   #15
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Quote:
Originally Posted by ZM2 View Post
Mine acted just like that, and then got worse and worse and started doing it all the time.
I see. I'd be very interested to see if your issue gets solved, and I will probably go the same route. Thanks for updating on here when you find out!
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      09-17-2022, 02:52 PM   #16
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Quote:
Originally Posted by ZM2 View Post
Pretty sure all these recent idle complaints are due to injectors getting older/used and either getting clogged up or beginning to leak.

Mine is in the shop for something similar and two plugs were black. Ran perfectly fine under load and at WOT, just a bad idle that got worse and worse over the last year. Kind of indicates leaking injectors in those cylinders, leading to rich mixture misfires at idle.

Doing new injectors early this week & will let you all know.
Oh, I just saw you in a previous thread from about a year ago mention that you replaced your MAF sensor and it helped this situation. Did it end up only being a temporary fix, and the cold start stumble eventually returned? I have also pondered just replacing the MAF sensor to see what happens.
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      09-17-2022, 02:59 PM   #17
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Quote:
Originally Posted by StateVector View Post
Oh, I just saw you in a previous thread from about a year ago mention that you replaced your MAF sensor and it helped this situation. Did it end up only being a temporary fix, and the cold start stumble eventually returned? I have also pondered just replacing the MAF sensor to see what happens.
Yeah, was just a temporary fix. Kind of like resetting adaptations, when the underlying issue hasn’t gotten that bad.
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      09-17-2022, 03:01 PM   #18
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Quote:
Originally Posted by ZM2 View Post
Yeah, was just a temporary fix. Kind of like resetting adaptations, when the underlying issue hasn’t gotten that bad.
I see, thanks.
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      09-23-2022, 12:37 PM   #19
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Quote:
Originally Posted by ZM2 View Post
Pretty sure all these recent idle complaints are due to injectors getting older/used and either getting clogged up or beginning to leak.

Mine is in the shop for something similar and two plugs were black. Ran perfectly fine under load and at WOT, just a bad idle that got worse and worse over the last year. Kind of indicates leaking injectors in those cylinders, leading to rich mixture misfires at idle.

Doing new injectors early this week & will let you all know.
How did your injector job go? Any improvement with stumble on startups? Mine goes into the shop for routine maintenance next Wednesday, so it might be something to add to the list..thanks for your feedback!
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      09-23-2022, 01:30 PM   #20
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Quote:
Originally Posted by StateVector View Post
How did your injector job go? Any improvement with stumble on startups? Mine goes into the shop for routine maintenance next Wednesday, so it might be something to add to the list..thanks for your feedback!
Hey, I don't have my car back, yet, but my mechanic says the idle is good now.

Plugs 3 & 6 were black, but the car ran fine with no misfires under WOT, so kind of points to an injector issue that was fouling those plugs and causing rough idle.

That said, we went back with all new plugs & coils while we were in there (free FCP replacement items), and the injectors, and all is good.
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      09-23-2022, 01:35 PM   #21
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Quote:
Originally Posted by ZM2 View Post
Hey, I don't have my car back, yet, but my mechanic says the idle is good now.

Plugs 3 & 6 were black, but the car ran fine with no misfires under WOT, so kind of points to an injector issue that was fouling those plugs and causing rough idle.

That said, we went back with all new plugs & coils while we were in there (free FCP replacement items), and the injectors, and all is good.
Thanks for the update! So my stumble just happens on the first cold start of the day and I had new plugs put in last year with no visible fouling on the old plugs. If the idle stumble and rev-up thing is due to leaky injectors, how would I determine which ones to replace? I assume you just replaced injectors 3 & 6 based on the fouling of those plugs? Maybe I should wait until my idle stumble thing gets worse and happens on every startup to do any mitigation?
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      09-23-2022, 01:45 PM   #22
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Quote:
Originally Posted by StateVector View Post
Thanks for the update! So my stumble just happens on the first cold start of the day and I had new plugs put in last year with no visible fouling on the old plugs. If the idle stumble and rev-up thing is due to leaky injectors, how would I determine which ones to replace? I assume you just replaced injectors 3 & 6 based on the fouling of those plugs? Maybe I should wait until my idle stumble thing gets worse and happens on every startup to do any mitigation?
We did all six injectors ($575): https://www.fcpeuro.com/products/bmw...-13647639994kt

Anytime I've played the which plug, coil, (and now injector) to replace game, the labor ends up being more if you don't get it right the first time.

And, my car has 40k miles, lots of track days, and years of running ethanol (also installed Bend's fuel filter), so probably better just to replace them all anyways.

Maybe a little more painful the first-time cost wise to do them all at once, but now that I'm in the FCP replacement cycle for those parts, we just replace them all each time to completely rule out plug, coil, and now injector issues.
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