Cold air intake causing rich condition

Question

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.

Answer 1

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.

Answer 2

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.

Answer 3

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.