This is a follow up question to my initial question which can be found here : LTFT and STFT values are off and don't make sense

Car is a 2015 F10 M5 Mods : Cold air intake, aftermarket rear muffler (just section 3).

Since posting the initial question, I have done a lot of research, learned a lot about the car and figured out what the problem was. The problem is, the conclusion I come to makes no sense.

So let me summarize:

  1. After doing several test drives, I now know that stock M5 from the factory has a 7-10% fuel trim variation between its both banks. I have confirmed this after driving 3 different cars built in different years. All these cars had Bank 1 running up to +10-12% and Bank 2 usually around +2% to 0%. So the issue in my car since day 1 had nothing to do with only one side. Why this is the case, I dont know, but I can speculate. The engine in this car is not symmetrically positioned. In other words, one side is lower than the other side (tilted) and everything is also slightly shifted (probably to make room). Intercoolers, wastegates, etc is all positioned slightly asymmetrical.
  2. The fuel issue I had was, my car was running -4% and -15% in bank 1 and bank 2. So compared to factory, that is 15% rich which is significant. This issue would happen mostly around idle and low rpms (below 4000). Highway mileage on my car (because you always drive on 7th gear and lowest rpm while cruising) was also impacted where I would barely maintain 19 mpg vs now I can easily keep 23.
  3. After removing my cold air intake, fuel trims immediately went back to factory values. Now I am getting 10% bank 1 and 0% bank 2.
  4. I have done multiple factory fuel trim adaptation resets before and after swapping intakes to confirm that none of these values are false positives.
  5. With factory software ISTA, I see that with my cold air intake, multiplicative mixture adaptation read 0.96 bank 1 and 0.85 bank 2. With stock intake, it reads 1.0 bank 1 and 1.10 bank 2. So i can confirm that my obdii tool and software I use (kiwi and dashcommand) accurately reports these values.
  6. With factory intake I feel the car has more punch in low rpms (higher low end torque) but significantly less power at higher rpms. Car was pulling much stronger with the cold air intake at high rpm.

So now after all of this, I am puzzled and this is driving me insane. How can a cold air intake cause a rich condition? Arent cold air intakes supposed to cause a lean condition not rich? Is there a plausible explanation to all of this?

Furthermore, is running cold air intake in my case going to have very long term effects? I want to keep this car for another 7-10 years. If the car was running 15% richer compared to stock, isnt that very bad (for injectors, coils, plugs, or just the engine itself) ? Or is this some software hoax somewhere? It is mind boggling to me because a cold air intake should not make a car run 15% rich. That is significant.

  • 2
    What intake did you install? Could the different air flow path cause issues for the MAF which would have been tuned specifically for the stock intake? The simplest explanation is that the MAF is not properly sensing the correct amount of air.
    – JPhi1618
    Feb 22, 2016 at 15:08
  • Its an intake called MSR. You can see the pictures here : 6post.com/forums/showthread.php?t=1095919. I thought about what you are saying but 2 things that dont make sense are : BMW uses the same bosch maf sensor on all turbo cars. I highly doubt they have a way of tuning the maf sensor. Part number is same across many cars. The other thing is positioning of the MAF sensor on this intake looks like right where the stock sensor sits. But even if you are right, how can the car feel more powerful with the intake?
    – DerStig
    Feb 22, 2016 at 15:16
  • I'm speaking in generalities because I'm not familiar with that car at all, but maybe someone here will have more experience. I wasn't thinking about tuning the actual electronic sensor of the MAF, but the position in the airflow and the turbulence (or lack thereof) of the flow can effect readings to a degree. With such a premium car and intake, does the manufacturer have any input? I'd certainly like to know about whatever you find.
    – JPhi1618
    Feb 22, 2016 at 15:33
  • DerStig, do you have any idea what kind of boost pressures the turbo runs, and whether wastegate pressure is electronically controlled?
    – Zaid
    Feb 22, 2016 at 21:09
  • This blog post seems to have good information on the turbo setup. May be useful to explain what's happening here.
    – Zaid
    Feb 23, 2016 at 11:32

3 Answers 3


I agree with @JPhi1618 that it is related to the MAF sensor. From the picture you provided in your comment, it appears your (really good looking) MSR intake comes with its own MAF sensor housing.

I too am unfamiliar with this car, but the MAF housing on your new intake is likely a different diameter than the OEM housing. This causes problems, as your OEM MAF sensor is programmed to read the volumetric flow rate (Q = flow velocity x cross-sectional area) for that specific tube diameter.

If the location of the MAF sensor in your new intake has a smaller cross-sectional area than the OEM MAF housing, your MAF sensor would be reading a higher flow velocity for the same amount of air. With the MAF sensor claiming it's getting a high flow rate, the ECU will compensate by dumping more fuel into the engine, causing the rich condition.

You might check to see if MSR sells a different set of MAF sensors, or look into getting aftermarket MAFs and have them tuned to match your new intake.

  • Hang on, when you say ECU would be dumping more fuel when air flow is higher, wouldnt that be a lean condition not rich? Remember the ECU must meet 14.7 AFR, if its suddenly getting more air, its going to add more fuel to match that AFR or else AFR would be above 14.7. Now more air means LEAN. This is where I am very confused:( Because I would expect my LTFTs to be much more positive (maybe 20%). Running Rich means ECU had to REMOVE fuel to meet that AFR.
    – DerStig
    Feb 22, 2016 at 15:57
  • 1
    @DerStig Moose is right, the ecu THINKS the airflow is higher because of the lower intake diameter (but it isn't) - lower cross-section causes higher airflow velocity and cools the MAF quicker. As a result the ecu compensates with more fuel, causing a rich condition. If the intake diameter really is lower, that is. Feb 22, 2016 at 16:05
  • @DerStig Would you be able to monitor the oxygen sensor values to confirm/deny that they stay about the same before/after installing the CAI? Given Moose's answer, I would think the O2 would report a higher A/F ratio (Leaner)
    – rpmerf
    Feb 22, 2016 at 17:05
  • Well, once trims stabilize, O2 sensors will (should) report identical values. Car does maintain perfect AFR. The right way to figure this out is to reset fuel adaptations while on a dyno, put CAI, test, reset again, put stock, test, and so on. But that is not something I can do. Technically I could do it myself but the install is fairly involved and every time I install/uninstall, chance of breaking something (ripping a coolant hose or cracking turbo inlets etc) goes up. My question is, what is the harm of running CAI in this car long term (7-10 years) knowing the rich condition?
    – DerStig
    Feb 22, 2016 at 17:15
  • 1
    At least on my M5, the IAT is used for other things. It's not involved in calculating mass air flow
    – Zaid
    Feb 22, 2016 at 21:06

I believe that what @JPhi1618 stated is a major factor in your rich condition:

Could the different air flow path cause issues for the MAF which would have been tuned specifically for the stock intake?

And that this statement that you made to counter I believe not to be true:

 BMW uses the same Bosch `MAF` sensor on all turbo cars. I highly doubt they have a way of tuning the MAF sensor

I am sure that while BMW may use the same MAF sensor in all turbo cars the ecu for each car is tuned differently on the MAF table and it is adjusted for each different intake setup.

Besides the changed intake shape, there could also be some other factors at play here. One would be any change in the actual air temp vs reported air temp used in fueling calculations. Let's say for simplicity sake that with the stock intake on, your air temp was reading exactly the same as the real temperature of the air entering the engine.

With the new intake the temp reading could be different from the air temperature entering the engine. Let's say your sensor is reading ambient air temp but your engine is sucking in hotter air. Your ecu will think its getting denser cold air and dump in a lot of fuel when in reality the air is not that dense.

In summary the following could be causing your rich condition:

  • the change in intake shape
  • a possible air temperature discrepancy

So, on to your second question about the risk of running rich for a long period of time. I will give a brief explanation of how the fueling logic works and you can decide.

  • First on cold start your car is probably running in open loop mode meaning that it is not using the output from the o2 sensors to do any fueling calculations it is just looking up a table which knows how much air will fit in the engine at a certain rpm and throttle position.
  • Second when your car is warmed up then it will start using the STFT and LTFT to augment the table I previously mentioned. So basically lets say your value for 100% throttle at 4000 rpms is 100 if your LTFT value for 100% throttle 4000 rpms is -10% then if you were to go into that table and take off 10% to make 90 your tune would be correct and next time your LTFT will be 0%. This is an over simplification because you don't really want to tune off LTFT but it is basically what is happening.

So what does this mean for your situation? Your ECU is basically saying "I would be running really rich here but I already know to take out x% of the fuel." I think where you will really have problems is if your LTFT numbers are not consistent. You also want STFT close to 0

This is how a lot of people run CAI's without ever re-tuning their ecu.

  • 1
    That's a great answer. Welcome to Stack Exchange. Hope to see you around some more with answers like this! Cheers. Feb 22, 2016 at 21:36
  • This explanation would work if the IAT is used to compute density. However, in Bosch fuel management they use it to control things like ignition advance. Hot-film MAF's are quite capable of determining mass flow without an intake air temperature sensor
    – Zaid
    Feb 22, 2016 at 22:08
  • Edited my answer to better fit this car's setup.
    – EricF
    Feb 22, 2016 at 22:59
  • I think overall we agree that changed intake shape and diameter will effect the actual density of the air vs calculated density of the air, causing the fueling calculation to be off.
    – EricF
    Feb 22, 2016 at 23:04
  • So you are saying basically intake is tricking the ECU into thinking more air is coming when that is not true
    – DerStig
    Feb 27, 2016 at 20:50

This isn't as scientific an answer as the other post, anyways, from the death throws of my 2000 SAAB 9-3, it is a lean condition. Oil from your intake filter got sucked onto the heat elements of the MAF, causing the computer to fail to get an accurate temperature reading. The car is compensating by adding fuel. This caused a MAF and Ignition Casette to fail years ago but the memory is still fresh.

  • My CAI does not use an oiled filter so this cannot be it. It uses a steel cone filter.
    – DerStig
    Feb 22, 2016 at 20:24

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