TL, DR: an aftermarket intake fixed my very high fuel trim issue but introduced a stuttering issue at low throttle input that was never present before. Doesn't make sense. Details follow:
For the past year, I've had high long term fuel trims on my M3. About a year ago, they were at 10-12%; I smoke tested and replaced fuel pump, FPR, fuel filter, and some other things but couldn't figure it out. Car drove ok so I let it go.
In the last month, I got a SES light and error code saying system too rich, checked fuel trims and there were at ~23-24%, so obviously the problem had gotten worse. Car still seemingly drove ok, SES light would go on and off from time to time.
Yesterday, I swapped my stock airbox for a prototype "cold air" intake I bought off a guy online (to make cool sounds). This is where it gets interesting:
Simultaneously, why would this change introduce a stuttering issue?? If fuel trims look good, it should be running better than before...
Final data point: I noticed via my OBD monitor that airflow at idle increased after the new intake from ~4 g/s to 5.4 g/s, as well as calculated load from 14% to 20%. Not sure if that matters. More data can be viewed here: https://datazap.me/u/mittortz/beforeafter-new-intake
Requesting insight from someone smarter than me.
For the past year, I've had high long term fuel trims on my M3. About a year ago, they were at 10-12%; I smoke tested and replaced fuel pump, FPR, fuel filter, and some other things but couldn't figure it out. Car drove ok so I let it go.
In the last month, I got a SES light and error code saying system too rich, checked fuel trims and there were at ~23-24%, so obviously the problem had gotten worse. Car still seemingly drove ok, SES light would go on and off from time to time.
Yesterday, I swapped my stock airbox for a prototype "cold air" intake I bought off a guy online (to make cool sounds). This is where it gets interesting:
- After a ~30 min drive with the new intake, I decided to check fuel trims. They had dropped to 5-7%, and as I kept driving, I saw them continue to drop to 3%. Whoa.
- For the first time ever, I noticed a distinct hesitation/stutter when cruising at very low throttle input. It felt like I was taking my foot off the throttle and putting it back over and over, but erratically.
- I did get a couple codes, INPA said 174 (system too lean) and 230 (Electronic Throttle Setpoint/Actual Value Deviation). Hard to tell if this was residual from the previous issue when I had very high fuel trims, so I cleared adaptions completely.
- Went on another drive, fuel trims looked great (1-2%) but the stuttering issue remained. This continued to happen with cruise control on the highway, so smooth roads and my foot wasn't actually on the throttle meant it was very distinctly a problem with the engine/car itself. No codes after driving for 45 minutes.
Simultaneously, why would this change introduce a stuttering issue?? If fuel trims look good, it should be running better than before...
Final data point: I noticed via my OBD monitor that airflow at idle increased after the new intake from ~4 g/s to 5.4 g/s, as well as calculated load from 14% to 20%. Not sure if that matters. More data can be viewed here: https://datazap.me/u/mittortz/beforeafter-new-intake
Requesting insight from someone smarter than me.
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