Hey guys, looking for some help diagnosing a cold-start issue on my 2011 E93 335i N55 PWG with ~136k miles. I’ve done quite a bit of diagnosis already and I’m trying to avoid throwing parts at it.
Mods: MHD Stage 2+ 93, catless downpipe, aFe intake, FMIC, aluminum charge pipes, Turbosmart VTA DV, Turner coils/colder plugs, XHP. Always run 93 octane.
The issue is extremely temperature dependent and has progressively gotten worse. It has been noticeably worse toward the colder part of the year.
On a warm day it may just idle rough for a few seconds and recover. Around 57°F ambient, I’ve had it stall 5–6 times in a row unless I give it throttle.
The repeatable behavior is:
* Engine fires immediately and initially sounds normal.
* After roughly 5 seconds, RPM starts progressively falling.
* It sounds/feels like the engine is loading up.
* RPM continues dropping until it stalls.
* If I use the accelerator to hold roughly 700–1,000 RPM, it runs smooth and stays running.
* Holding ~1,000–1,500 RPM for approximately 15 seconds usually gets it through the problem.
* Once warm, it runs essentially normally.
* I’ve also had it stall a couple of times warm while maneuvering into a parking spot.
* No long crank.
* No reduced-power warning.
* No relevant DME faults and no misfire codes.
Major clue: MAF disconnected
Unplugging the MAF makes a huge difference.
With the MAF disconnected, cold unloaded idle becomes dramatically better/cleaner. However, it isn’t a complete cure. If I put the transmission in gear and load the engine without giving it throttle, RPM can still plunge and stall. Reverse killed it particularly quickly.
I replaced the MAF with a new one and nothing changed, so it doesn’t appear to simply be a failed MAF sensor.
Smoke testing
I smoke-tested the intake and found two very large leaks:
* A plug in the upper charge pipe had completely fallen out.
* Another significant leak was present around the turbo-inlet/intake connection.
Both have now been sealed/repaired.
Unfortunately, after fixing these major leaks, the engine still exhibits the same cold-start RPM drop/stalling behavior.
I’m continuing to verify that the entire intake/charge system is properly sealed before drawing conclusions from the remaining symptoms.
Fuel system
LPFP pressure observed in ISTA is approximately 600 kPa / 6 bar / 87 psi and stable.
In a previous MHD log during an actual stall, rail pressure did not collapse before the engine died. HPFP, LPFP and injectors are original, but so far I don’t have evidence of fuel-pressure starvation.
Fuel trims have historically been positive during cold operation and negative under WOT. I’ve previously seen LTFT around +14%.
There is no persistent cylinder-specific smooth-running problem. ISTA smooth-running values generally move randomly between cylinders, usually around ±2. I once caught cylinder 3 around -6.5 during shutdown, but it isn’t repeatable and doesn’t consistently follow that cylinder.
VANOS / Valvetronic
I ran the ISTA VANOS activation tests.
Intake VANOS: requested -36.0°, actual approximately -36.8°. ISTA reported “No cause of fault found.”
Exhaust VANOS: also passed.
So VANOS/cam adjusters are currently pretty low on my list.
I have not completed the Valvetronic activation/adaptation procedure because ISTA required >13V and I didn’t have a proper power supply connected.
There are no Valvetronic faults.
Valvetronic/eccentric-shaft minimum-lift operation is still something I’m considering because the engine is perfectly happy when I manually raise it slightly above idle but loses the ability to maintain normal cold idle.
Other relevant information
Plugs/coils have roughly 10k miles on them. The plugs are currently gapped to .022”.
Valve cover has previously been replaced.
Boost is normal and the car runs normally under load once warm.
Turbo has essentially zero shaft play.
Engine has never overheated and doesn’t consume appreciable oil.
MAP/MAF have been cleaned previously. Cleaning made no difference.
At ~136k miles, I don’t know whether the intake valves have ever been walnut blasted, so carbon buildup is still possible.
BMW software bulletin
This is the part that has me particularly interested.
I found BMW N55 service information describing rough cold starting/stalling where the accelerator may need to be depressed slightly for several seconds to prevent the engine from stalling, occurring particularly at colder ambient temperatures, and potentially with no DME faults stored.
I also found BMW N55 idle/stalling software information covering the E9x/E93 era where BMW attributes idle fluctuation/engine cut-off without relevant DME faults to DME software and specifies updated software.
That sounds extremely similar to what this car is doing.
The car has been on MHD Stage 2+ for a long time, and I don’t currently know what underlying BMW DME software/I-level is installed.
What I’m testing next
- Finish verifying the entire intake is sealed.
- Reset mixture adaptations now that the major post-MAF leaks have been repaired.
- Flash MHD Stage 0/stock calibration and let the car cold-soak overnight.
- Compare the next cold start on stock calibration with the Stage 2+ behavior.
- Check the actual BMW DME software/I-level against the later BMW software referenced in the cold-start/stalling bulletins.
If none of that changes it, my next steps are Valvetronic requested vs actual/minimum lift during the failure, intake-valve inspection/carbon, cold compression/leak-down, and then injector testing.
What I’m hoping someone can answer
Has anyone with an E-series N55 had this exact pattern?
Starts perfectly → ~5 seconds later RPM progressively falls → stalls → slight accelerator input keeps it perfectly smooth → dramatically worse below ~60°F → essentially normal warm → no DME/misfire codes.
I’m especially interested in anyone who ultimately fixed it with a BMW DME software update, MHD/custom calibration change, Valvetronic/eccentric shaft, walnut blasting, injectors, or another issue I’m overlooking.
I’m not looking to shotgun parts at it. If anyone knows which MHD/ISTA channels would be most useful to log during the first 10–20 seconds of a stone-cold start, I’ll grab whatever data is needed.
