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2Addicts | BMW 2-Series forum Technical Topics N55 (M235i) Engine, Transmission, Exhaust, Tuning Stage 2 FBO N55 Backfire at idle and boost cuts WOT

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      08-15-2022, 08:49 PM   #1
Cade.kramer
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Stage 2 FBO N55 Backfire at idle and boost cuts WOT

My 15' m235 has been perfect from the start but recently started to pop a bit at idle. I also noticed under heavy boost at lower rpm's, there would be a noticeable boost cut and noise that is somewhat like an up shift on a dsg trans. It backfires/farts for a second and then kicks back in and is usually fine for the rest of that pull. Spark plugs are one step colder and less than 5000 miles old but the plugs are old. My mechanic is saying not to replace plugs until I throw codes for it. I think it feels like something fuel related but I'm not positive. I'll attach my logs from today and if anyone has any advice that would be much appreciated.

IDLE: https://www.bootmod3.net/log?id=62fa...0b432dfabc757c

DRIVING IN SPORT+: https://www.bootmod3.net/log?id=62fa...0b4330beb553cd

Sorry for the absolute clutter, as I wasn't sure what to log data for exactly I selected almost everything. If it's easier for me to do another log with less selected let me know.
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      08-28-2022, 03:53 PM   #2
MadJonny
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Did you solve the issue? My car did a similar thing a couple of weeks ago. I changed the sparkplugs after (4000 km / 2500 Miles) and it was ok again. Mind the plugs were not correctly gapped so I guess that was the issue.

I wouldn't drive the car this way as missfiring under load isn't really great for an engine and often it takes some time for the engine light to light up even though there is a serious problem.
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      09-05-2022, 01:24 AM   #3
rtl32
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You're dealing with two different problems.

In your sport+ log the biggest pull shows a 5-600psi fuel pressure dip which is when you would've experienced a backfire/fart/power cut. Very normal on these cars, very common issue, the fuel pump won't keep up with a tune. Just put a HPFP on and it'll go away. My OEM HPFP wouldn't even keep up with my car on a stage 1 tune. It even did this on the stock tune a couple times as well.

Now for the idle problems 98% of them are caused by bad/mis gapped plugs or bad coils, and the other 2% are caused by messy and partially stuck injectors. I did my injectors at 50k and my plugs every 10k and my coils every 15k and it keeps everything fresh. If its not one of those things you have an air leak but that would throw codes.

Upgrade your HPFP or go back to stage 1, and make sure your plug gap is okay, your coils are good, and if you're still having problems maybe do injectors but check back for that because you would be having other problems if thats the issue as well (namely low load throttle transitions will get stumbly, cold starts might be rough for the first 2-5 seconds, and you'll have some timing corrections isolated to cylinders after you've already confirmed coil/plug are good if injectors are the case).
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      09-07-2022, 03:17 PM   #4
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rtl32

Awesome troubleshooting advice. Do you think cleaning and/or resetting the maf sensor to be beneficial in this instance? I speak in regard to the idle issue.

Pressure dip would be addressed with hpfp as you noted.
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      09-14-2022, 09:03 AM   #5
MadJonny
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Quote:
Originally Posted by rtl32 View Post
You're dealing with two different problems.

In your sport+ log the biggest pull shows a 5-600psi fuel pressure dip which is when you would've experienced a backfire/fart/power cut. Very normal on these cars, very common issue, the fuel pump won't keep up with a tune. Just put a HPFP on and it'll go away. My OEM HPFP wouldn't even keep up with my car on a stage 1 tune. It even did this on the stock tune a couple times as well.

Now for the idle problems 98% of them are caused by bad/mis gapped plugs or bad coils, and the other 2% are caused by messy and partially stuck injectors. I did my injectors at 50k and my plugs every 10k and my coils every 15k and it keeps everything fresh. If its not one of those things you have an air leak but that would throw codes.

Upgrade your HPFP or go back to stage 1, and make sure your plug gap is okay, your coils are good, and if you're still having problems maybe do injectors but check back for that because you would be having other problems if thats the issue as well (namely low load throttle transitions will get stumbly, cold starts might be rough for the first 2-5 seconds, and you'll have some timing corrections isolated to cylinders after you've already confirmed coil/plug are good if injectors are the case).
Hi rtl32
Why do you think this is related to HPFP not giving enough fuel to the engine because as you climb RPM you can see there is no dip between target and actual? Couldn't it be more related to LPFP?

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      09-16-2022, 12:21 AM   #6
rtl32
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Quote:
Originally Posted by MadJonny View Post
Hi rtl32
Why do you think this is related to HPFP not giving enough fuel to the engine because as you climb RPM you can see there is no dip between target and actual? Couldn't it be more related to LPFP?

Attachment 2982298
Because that is the issue 98% of the time on these cars.

Looking at your log (its better to post the bm3 link than a screenshot so I can actually see the values) I can see 3 hpfp dips mid pull. HPFP dips are so common on these engines it's predictable.
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      09-17-2022, 10:36 AM   #7
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Quote:
Originally Posted by rtl32 View Post
Because that is the issue 98% of the time on these cars.

Looking at your log (its better to post the bm3 link than a screenshot so I can actually see the values) I can see 3 hpfp dips mid pull. HPFP dips are so common on these engines it's predictable.
Hi rtl32, This is a screenshot of Cade.kramer (OP) and was looking for what you mentioned on your previous post
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      09-18-2022, 09:29 PM   #8
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Quote:
Originally Posted by MLJR235 View Post
rtl32

Awesome troubleshooting advice. Do you think cleaning and/or resetting the maf sensor to be beneficial in this instance? I speak in regard to the idle issue.

Pressure dip would be addressed with hpfp as you noted.
I have cleaned my MAF sensor many times (every oil change and when I was troubleshooting a lean code thrown by an old Dinan intake I used to have). I noticed a slight improvement in idle and cold starts immediately after cleaning but I attribute that to the car running open loop temporarily after losing the MAF sensor value because it was unplugged.

A lot of guys say that unplugging and reconnecting their MAF improves their idles and fixes some cold start issues - but I think its a temporary coverup and symptom vacation for when something isn't doing what the car expects it to (plugs, coils, injectors). The MAF affects the car but not that much. A lot of big turbo guys especially on N54 and in the JDM world run completely MAFless and are completely fine. I get why people want to focus on the MAF its easily accessible, cheap and easy to clean but it's also really far down the list of what will actually cause problems (unless you have an intake tube that is a different diameter than stock, then it will cause problems because its reading a hugely different flow than what is actually occurring)

In other words what playing with the MAF actually does is switches these cars to a different fueling method that is less sensitive to other aspects of the system that may not be working correctly.
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