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      11-20-2017, 04:44 PM   #1
Racer35
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6MT Erratic Idle

Has anyone else had an issue with erratic idle (with a stock ECU)? My 6MT is about a year old, with 17K miles, and I've noticed a stumble at stop lights over the last couple months. Idle oscillates between 500 and 750 RPM, stumbling at 500. It was only occasional a couple months ago but it happens every time now. This happens with all ECU modes and doesn't seem to have anything to do with where I buy my gas. Until recently, idle was dead-nuts-steady. Wondering if I have a bad sensor somewhere.
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      11-20-2017, 06:17 PM   #2
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Quote:
Originally Posted by Racer35 View Post
Has anyone else had an issue with erratic idle (with a stock ECU)? My 6MT is about a year old, with 17K miles, and I've noticed a stumble at stop lights over the last couple months. Idle oscillates between 500 and 750 RPM, stumbling at 500. It was only occasional a couple months ago but it happens every time now. This happens with all ECU modes and doesn't seem to have anything to do with where I buy my gas. Until recently, idle was dead-nuts-steady. Wondering if I have a bad sensor somewhere.
I recently had to have a fuel pressure sensor/module replaced on my 2015 with 22K miles.....my car showed some similar symptoms and then a few weeks later sputtered and died at a stop light, and it took me a few tries to get it started again....and I got a Drivetrain Malfunction error because of it.
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      11-21-2017, 10:09 PM   #3
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I feel like this happened to my car once when it was about 2 months old (M2). Almost died, but then idled fine and hasn't done it since. Never had it checked out, but still odd.
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      11-22-2017, 01:14 AM   #4
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Quote:
Originally Posted by jasnh View Post
Quote:
Originally Posted by Racer35 View Post
Has anyone else had an issue with erratic idle (with a stock ECU)? My 6MT is about a year old, with 17K miles, and I've noticed a stumble at stop lights over the last couple months. Idle oscillates between 500 and 750 RPM, stumbling at 500. It was only occasional a couple months ago but it happens every time now. This happens with all ECU modes and doesn't seem to have anything to do with where I buy my gas. Until recently, idle was dead-nuts-steady. Wondering if I have a bad sensor somewhere.
I recently had to have a fuel pressure sensor/module replaced on my 2015 with 22K miles.....my car showed some similar symptoms and then a few weeks later sputtered and died at a stop light, and it took me a few tries to get it started again....and I got a Drivetrain Malfunction error because of it.
That sounds about right. I don't have a malfunction light yet, but haven't had it stall... Hopefully they'll be able to diagnose it easily without error codes.
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      11-22-2017, 09:40 AM   #5
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Quote:
Originally Posted by jasnh View Post
I recently had to have a fuel pressure sensor/module replaced on my 2015 with 22K miles.....my car showed some similar symptoms and then a few weeks later sputtered and died at a stop light, and it took me a few tries to get it started again....and I got a Drivetrain Malfunction error because of it.
Dang, similar issue with low-pressure fuel pump sensor ala N54.
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      11-22-2017, 10:30 AM   #6
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Dang, similar issue with low-pressure fuel pump sensor ala N54.
Yeah, sounds like HPFP and/or injector banks similiar to what they had with the N54 you reference above. Which BMW did resolve with replacements of the items above and software adjustments.

OP, as stated it can leave you high & dry at a stop light without notice once these symptoms start, thus getting it looked at by BMW sooner than later is prudent.
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      11-22-2017, 12:16 PM   #7
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Yeah, sounds like HPFP and/or injector banks similiar to what they had with the N54 you reference above. Which BMW did resolve with replacements of the items above and software adjustments.

OP, as stated it can leave you high & dry at a stop light without notice once these symptoms start, thus getting it looked at by BMW sooner than later is prudent.
not 100% sure, but his issue seems to be not HPFP/injectors but the sensor that monitor the low-pressure fuel pump, LPFP. on N54 it went bad, reads high so LPFP backs-out in reality starving the HPFP, engine stalls, sometime on low speed. Very scary situation on N54, if you happens to turn left across oncoming traffic, it happened to me. it's $75 sensor under air-intake on N54.
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      11-22-2017, 01:01 PM   #8
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Quote:
Originally Posted by HPIA4v2 View Post
not 100% sure, but his issue seems to be not HPFP/injectors but the sensor that monitor the low-pressure fuel pump, LPFP. on N54 it went bad, reads high so LPFP backs-out in reality starving the HPFP, engine stalls, sometime on low speed. Very scary situation on N54, if you happens to turn left across oncoming traffic, it happened to me. it's $75 sensor under air-intake on N54.
...probably right but in any event getting it checked before it puts you in a dangerous situation is good advise.
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      11-23-2017, 02:32 PM   #9
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Alright... I was hoping to wait until my next oil change (2,000 miles away) but I don't want it to leave me hanging. I'll get it scheduled and report back. Thanks for the advice.
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      11-28-2017, 01:12 AM   #10
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Quote:
Originally Posted by HPIA4v2 View Post
not 100% sure, but his issue seems to be not HPFP/injectors but the sensor that monitor the low-pressure fuel pump, LPFP. on N54 it went bad, reads high so LPFP backs-out in reality starving the HPFP, engine stalls, sometime on low speed. Very scary situation on N54, if you happens to turn left across oncoming traffic, it happened to me. it's $75 sensor under air-intake on N54.
I went through so many sets of injectors and fuel pumps in my E92 335i I loved that car, but couldn't afford the risk once the warranty was over.

Another potential culprit for erratic idle is a faulty or dirty MAF.
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      12-22-2017, 01:21 PM   #11
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For the record, mine turned out to be a bad ignition coil. Easy fix.
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      12-29-2017, 10:28 PM   #12
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Well, I took my car in 7 days ago and it's still there... Dealership was able to reproduce the problem and ID error codes but every time they test those specific parts, they come out clean. I didn't get the specific codes or parts out of them but there was reference to coils and the fuel system (not surprising)... They've contacted BMW NA to escalate the issue.
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      12-30-2017, 01:08 PM   #13
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Just got a call from the dealership. They found the problem and it's been repaired. There was a tiny leak in the intercooler that was causing pressure variance, resulting in fuel trim oscillation. They did a smoke test on the intercooler to find the leak.
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