N55 Code P0171 Bank 1 Too Lean with Cold Air Intake

[makke]

This is one of those things that is totally obvious in retrospect and you feel completely stupid for not having thought of it earlier. However, it was not obvious to a number of smart people until, to use a football metaphor, Monday morning. Posting to, hopefully, save others time and heartburn in the future.

For the past couple of months, have been struggling with a lean condition on my 2012 135i with 50K on the clock. Have had the car for a year. Previous owner installed an AFE cold air intake and an ER charge pipe. He was also running a JB4, but I am not a fan, so I took it out.

In the last two months, the car started throwing an occasional CEL with P0171, Bank 1 Too Lean. To help troubleshoot, I got an UltraGauge that can monitor OBDII parameters in real time. (http://www.ultra-gauge.com/ultragauge/) The ones I was specifically interested in were Short Term and Long Term Fuel Trim (STFT/LTFT). Sure enough, the ECU was adding quite a bit of fuel, with long term trim consistently above 20%, often at 24/25%. The code would get thrown when the ECU could could no longer add enough fuel to maintain appropriate fuel to air mixture.

https://photos.app.goo.gl/nUfYuN5wu5JTqiMy8

Researched this online and talked about this with several car guys at work. P0171 seemed to be one of those generic codes that could be caused by anything from an intake vacuum leak to a bad MAF to a bad O2 sensor to problems with the high pressure fuel pump...

Starting at the cheap end of the spectrum, took the car in to get smoke tested. Mechanic found a leak at one of the plumbing joints and replaced the seal. $160 later, I still had LTFT at 24%... The "fix" made no difference. His next suggestion was to replace the MAF (mass air-flow sensor). Well, the non-BMW part, alone, is $200. The BMW part is $400...

This past weekend I removed the MAF and cleaned it with CRC mass air cleaner. This seemed to make only a minimal difference. Next, I removed the MAF and MAP (manifold absolute pressure) sensors, and cleaned them both. I also had an epiphany and replaced the AFE intake with stock (previous owner kept the parts). Voila, Lng tr1 = 0.00:

https://photos.app.goo.gl/1dpfpGr3rxLFhsxXA

As I said, this is totally obvious in retrospect. The AFE intake is less restrictive, so the engine was sucking in more air than expected. The ECU compensated by adjusting the fuel trim. The MAF and/or MAP got a little bit of deposits on them over time and skewed the balance just far enough for the car to start throwing P0171. At least that's my best theory as to why things were fine for a year before the CEL episodes.

The take-away is that swapping the stock intake with a less restrictive after-market version will throw your car out of calibration. Therefore, a stage 1 tune is actually necessary to adjust for this. In my, very humble, opinion, intake swap has to be done in combination with a corresponding tune.

I hope this will be of value to someone.

[iminhell1]

or the meth bungs are leaking ...


and the more air theory isn't what's going on here.

[juld0zer]

Wasn't this issue with this particular intake discussed in depth earlier?

[makke]

iminhell1: I am not talking about the charge pipe -- just the intake. Like this one: (https://afepower.com/afe-power-54-11...-intake-system). Meth bungs are on the charge pipe.

juld0zer: You are right -- there are a number of discussions from around 2011 on e90post. However, they do not causally link the cold air intake to high fuel trim and lean condition. Also, because I bought the car with the intake already installed and did not have any problems for a year, I did not know that the intake was responsible. So I kind-of arrived at the whole thing from the opposite direction compared to most people, if that makes sense...

[Senhorcheeks435]

I'm having the same problem with my 2015 435 .. got Dinan intake vrsf charge pipe down pipe and I'm getting this code I already replaced hpfp .. I ordered a new maf I hope this helps 😔

[Alturiak]

Did your new MAF end up solving the issue? I'm in the same boat...

[wheela]

It is the intake causing the problem, but it's not actually consuming more air. Only a small fraction of air going through the intake goes through the MAF sensor. The DME has HFM (Hot Film Massflow) tables that correlate the flow through the MAF to the total flow through the crossection of air duct where the MAF is installed. When you change the duct where the MAF is located, you change the proportion of total air flow that flows through the MAF. So now your MAF sensor no longer reads correctly and as a result the DME thinks your're flowing much less air into the engine than you actually are. So it calculates a smaller amount of fuel to inject to hit AFR based on the erroneous MAF reading. Then your O2 sensor detects the lean condition, and has to add a ton of fuel trim to get the AFR back on target.

To fix the problem, you can either put the stock intake back on and leave your tune alone, or keep your aftermarket intake and custom tune your HFM tables to recalibrate your MAF sensor to read correctly in the new intake pipe.

[Peplano]

Wanted to mention on here, and sorta revive the thread. I also had this lean condition with a K&N drop in air filter. Firstly, after installing the new OEM intake, the car had seemed to respond to throttle, boost and everything a lot better. Secondly, my lean code had gone away after doing several pulls in the vehicle. Pending results after the next few days but this may be a leading resolution to lean codes.

[vermilyeabmw]

I also got this code on my N55 but it occurs infrequently and most recently I noticed it was a particularly hot day (meaning thinner air, so perhaps more airflow). I am suspicious of the drop-in washable aftermarket air filter installed two years ago as per the previous post, so I replaced it with a stock AF and will see if that solves the issue.