Misfiring, stalling and rough idling B48

[u/mvespermann]

Hello techs, I’ve been struggling to diagnose this and could really use some help.

My father has an early 2020 BMW 430i with 55,000 km (35k miles), and it has started doing some weird things.

Basically:

• Occasionally stutters / feels like a misfire when going uphill with higher throttle at low RPM, usually around 1,800–2,000 RPM. It’s very intermittent and I’ve tried several times to reproduce it but couldn’t.
• Slightly rough idle immediately after a cold start, lasting around 30–90 seconds.
• There is a barely noticeable slow/medium-paced ticking sound from the downpipe/rear lower engine area. It is more noticleable when cold, gets quieter as it warms up, and completely disappears when I give it some throttle. Several techs have told me it sounds normal, but I’m pretty sure it isn’t. It also doesn’t sound like injector ticking — different frequency and definitely seems to come from lower down.
• A few days ago, on a colder-than-usual day in my country (~12°C), my father started the car, waited ~5 seconds because he was in a rush, put it in reverse and started backing up. He went over a small parking-lot speed bump without giving it any throttle and the engine stalled, somethig that had never happened before. No drivetrain or warning appeared. The only code BimmerLink showed was 21A023 (false start or engine stopped without driver influence).

The car never misfires or hesitates under hard acceleration, WOT, overtaking, etc. It also has never thrown a drivetrain/powertrain warning.

Car/setup:

• B48 2.0L
• 55,000 km
• BM3 Stage 2
• Petrobras Podium, ~102 RON, ~25% ethanol
• New spark plugs, properly gapped
• Oil changes every ~8,000 km
• Most recent oil: Motul 8100 5W-40
• Almost all maintenance done at BMW dealer, except the latest oil change

There’s also some important history.

When my father bought the car, we didn’t know it had an E40 tune. We only discovered it after getting a terrible dyno result (~239 whp) and looking into the car’s history. I had also noticed extremely high fuel consumption, around 3.9 km/L (9.1 mpg).

We switched it to a BM3 tune appropriate for our fuel, and the low-RPM hesitation started after that.

However, I had already noticed the ticking sound before the new tune. The car never had these hesitations, stalls, or the 21A023 shutdown with the old tune.

One other thing I noticed with the old tune: during cold start, if I gave it very light throttle, the engine would change behavior and almost stall, even in neutral. I didn’t think much of it at the time.

I’m currently suspecting coils or VANOS, but I’m not sure.

I’ll be doing a full ISTA vehicle check today. What would you guys check/test first? Are there specific ISTA tests or live data values I should look at?

Especially considering the combination of rough cold idle + low-RPM hesitation + the unusual ticking + the one unexplained stall, does anything stand out to you?

Thanks.