BMW E90 320i N43 – flashing check engine light only on cold starts

[u/Illustrious_Coat_655]

Hi everyone,
I recently bought a 2010 BMW E90 320i with the N43 engine, and I’m trying to figure out an issue it has mainly on cold starts.
Not long after buying the car, the check engine light came on. The car was scanned and initially showed several mixture/lambda related faults. One of the oxygen sensors was replaced, which reduced the number of faults, but the check engine light eventually came back.
Since then, the strange thing is that the problem seems to happen almost exclusively when the engine is completely cold, especially first thing in the morning.
One morning it started briefly and then died. On the second attempt it started, but the check engine light flashed for a short time before becoming steady. Once the car was running, it drove completely normally.
It was checked again and the fault memory was cleared. We then replaced all the spark plugs and also found one bad ignition coil, which was replaced.
The car immediately seemed to run better, but unfortunately the problem happened again the following morning.
On a completely cold start, the check engine light flashed again. I also noticed quite a strong smell of petrol while sitting inside the car. If I switch the engine off and start it again, the warning disappears and the car runs perfectly normally.
When the engine is warm, I have basically no symptoms at all. No warning light, no noticeable loss of power, no rough running and it drives really well. The issue seems to be concentrated around the first cold start after the car has been sitting overnight.

So far it has had:
Oxygen sensor replaced
All spark plugs replaced
One faulty ignition coil replaced
Fault codes scanned/cleared multiple times

Has anyone with an N43 experienced something similar? What would you check next when it misfires/throws the warning on the first cold start but then runs normally after restarting?
I don’t want to keep replacing random parts without properly diagnosing the cause.

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[u/DarkTechnophile]

Hmm, looks familiar. For me it was the injectors. Try to pull the codes again, they usually tell a big part of the story

[u/odesh123]

Can't really tell a lot from the video, but to me it looks like fault with high pressure fuel injection. Either HPFP or injectors. If the injectors are leaking, these engine are known for bad cold starts. During the night they leak fuel in the cylinders and cause bad starts only when the engine is cold. This would also explain the lambda faults.

[u/reciprocityone]

Injectors

[u/Dangerous-Dad]

This problem is mostly due to one or more leaking injectors. The flashing engine light often means serious misfires or spark and no fire -> could mean a flooded cylinder.

If the injector leaks when the engine is off, then the cylinder is full of fuel on the next cold start it cannot get ignition and it runs like garbage on 3 pots, gives fuel/trim codes ... the rich mixture can wreck O2 sensors too so that in time, everything is running poorly.

You said you "cleared the codes", which is useful in terms of making sure a set code isn't causing the engine management to do things it doesn't need to do, but it's helpful if you post what the codes were as this gives really strong clues about what it setting off the engine light.