[Nbustard]

Looking for some insight from the N55 crowd. I’m chasing a cold-start idle instability / RPM hunting issue that has persisted despite a lot of recent maintenance, and I’m trying to narrow this down logically instead of throwing parts at it.

Vehicle Info
2013 BMW 335i E93
N55
~115k miles
Aftermarket chargepipe
VRSF high-flow catted downpipe

Software has mostly been stock / Stage 0. I briefly tested Stage 2 recently, but the issue predates any Stage 2 tuning, so I don’t believe this is tune-induced.

Symptoms

- Rough idle primarily when engine is cold

- Idle RPM oscillation / hunting when lightly applying throttle during cold operation

- Condition improves significantly once fully warm

- No hard fault codes currently present

- No misfire behavior under load

I have had vacuum leak codes and misfires in the past, but no misfire codes are currently present, and misfire counters do not increment in logs.

Log observations (multiple logs, stock software)

- STFT and LTFT trend positive at idle and light load

- AFR follows lambda target

- Rail pressure meets target (no HPFP dropouts)

- No misfire counters incrementing

Issue is present on stock software, further suggesting a mechanical or airflow-related cause rather than tuning.

Maintenance History

Completed within the last ~15k miles:

- OEM HPFP

- OEM water pump

- Valve cover (quality replacement, not low-end aftermarket)

- Oil filter housing gasket

- Oil cooler gasket

- Serpentine belt

- Belt tensioner

- Valve cover

Completed within the last ~2k miles:

- Spark plugs

- Ignition coils

- Oil change

Oil changes have been consistent, and oil consumption is normal.

Additional Details

- Valve cover was replaced, but PCV hoses / crankcase vent lines were not replaced

- No obvious boost leaks

- No drivability issues once warm or under moderate/heavy load

Current Hypothesis

Given:

- Cold sensitivity

- Positive fuel trims at idle/light load

- Idle instability without misfires

- Presence on stock software

- Healthy HPFP/injector behavior

I’m leaning toward a small unmetered air issue or crankcase ventilation problem, such as:

- PCV hose / crankcase vent leak

- Crankcase over-vacuum

- Intake air leak that seals once warm

- Throttle adaptation / airflow modeling issue after recent hardware work

I’m planning an intake + crankcase smoke test along with a crankcase vacuum check, but wanted to see if anyone has experienced similar behavior tied specifically to PCV hoses or less obvious air leaks on higher-mileage N55s.

Looking for input on:

- Commonly missed PCV hose / vent line failures on N55 even with a newer valve cover

- Other known causes of cold idle hunting with positive trims

- BMW-specific diagnostic steps worth checking before replacing parts

Appreciate any insight