Cannot Solve N55 Rough Idle

MatthewLC

New Member
Mar 23, 2019
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BMW F10 535
I'm looking for some help to solve a rough idle issue I've had for a long time.

Details on the car:
  • Early F10 N55 with rotary n54 style HPFP
  • Only mod is VRSF Catless downpipe and OEM M5 Mufflers
  • Coils/plugs/injectors are relatively new (OEM BMW parts)
  • Stage 2 MHD 91 Octane tune at the moment

In the attached log you can see the following behavior:
  • RPM at idle is not consistent, fluctuates and almost sounds like a missfire at idle
  • STFT oscillations at idle, STFT keep swinging back and forth and do not stay consistent
  • When car gets up to full operating temperature, it consistently goes into fuel mode 22 at idle. Not sure if this is normal? I can only find fuel mode description for N54, but if they are the same this is injector adaptation mode? Every time it goes into fuel mode 22, rail pressure spikes and car has a similar sound to "cat heating" cycle running (sounds like the engine loads itself up)

Curious if fuel mode 22 is normal? To me it seems like some sort of oem BMW catalyst strategy that isn't disabled? Just a guess at this point but the car doesn't seem happy to be catless and seems like it's going nuts at idle trying to figure out why the cat may be missing?

Anyone have any experience with this? I've got to a point where I may just put the OEM cat back on to see what happens?

Things I've done:
- Smoke tested (no leaks)
- Carbon Cleaning
- New coils
- New injectors (thought they may be leaking at idle)
- New plugs ( I've tried NGK, Bosch and OEM BMW Bosch)
- Tried flashing back to stock and multiple different ots tunes with same behavior

Car drives absolutely normal other than rough idle. It's been doing this since I've had the car (2+ Years). Tune and catless downpipe were installed the 2nd week I owned the car so I don't recall if it did this when it had the oem cat installed to determine if it's a hardware issue?

Here is a log: https://datazap.me/u/mcristiano/rough-idle?log=0&data=18-19-42

Video of what is sounds like:
 

wheela

Lieutenant
Jun 4, 2021
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Twin Cities, MN
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2015 e84 X1 35i Msport
Have you tried reading codes with MHD? I've tried looking up n55 fuel modes in the past, but have had no luck finding information. I suspect they're different than n54, because the DME's are different manufacturers (Bosch vs. Siemens). That said, I don't know what fuel mode 22 means, but it isn't normal. In my logs where I've monitored fuel mode i've always been 0 (I'm 9e60b) Looking at others logs, I recall they've been different than mine, but think they were either 1 or 2 - I don't recall seeing any logs with fuel mode = 22. Also, not sure if fuel modes differ between n55 roms or not, but it seems most n55 logs I've seen people post up were 98g0b, so not sure if that could account for my fuel mode being different.
 

MatthewLC

New Member
Mar 23, 2019
3
0
0
Ride
BMW F10 535
Appreciate the input!

There are no trouble codes unfortunately, so I really don’t have much direction. This has lead me to fire the parts cannon at this thing without much help.

If I’m not mistaken the rom on these early F10’s is 07D0.

Other than idle the car seems to either be in fuel mode 2 (which I believe is expected) and fuel mode 20 under harder acceleration.

Forgot to mention, pre and post cat o2’s are new. Also thought it may be a valve sealing issue considering it’s at idle only. Compression test came back perfect. Valve cover was also replaced (so I don’t think it’s PCV related and passed smoke test).

I figured it may be worth just going back to stock to rule out my theory (which is more of a guess at this point lol). A part of me also wonders if this could be worn valvetronic behavior? It’s original with 162k miles at this point.
 
Jun 2, 2022
22
20
0
is it normal with the stock tune/stock maps? I’d so then at least you’d be able to narrow it down to a software issue vs a hardware issue

The log is just at idle so try running completely stock and see what changes if anything.

Edit: sorry it looks like you may have done this already. This is strange. What flashtime options are selected? This might, at least, eliminate further the tuning piece depending on which options you’ve turned on.
 
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