Several readers of my blog have asked the following questions:
a. Is it normal if, directly after starting, the performance of the engine is a bit unstable, idle – increased;
b. What are the parameters of this after-start stage, and when it’s time to start to worry?
At first, the primary technical information: directly after the start, for the first 60 .. 90 seconds, DME is forcibly heating up the CO catalytic converter. To perform this operation, the following actions are taken:
a. increased idle is maintained (around 1000 RPM);
b. the exhaust VANOS has turned in such a position that exhaust valves are opening when the fuel combustion is still in progress in the combuston cycle.
Accordingly – the fuel combustion continues in the CO catalytic converter, and its temperature is quickly increasing. As we can guess, the heating of the CO catalytic converter is intended to reduce the amount of harmful emissions.
F series vehicles, which are equipped with a B58 engine, don’t have the temperature sensor of the exhaust system. The exhaust temperature is simulated (virtually calculated by the DME). It looks like BMW AG engineers don’t trust their own simulation software because the DME stubbornly repeats the heating cycle, even if the heated-up engine (and exhaust) is restarted several times.
It goes without saying that during the warm-up cycle, the engine does not work in a “normal” mode. The spreading of the flames during the combustion process is quite different from cycle to cycle. If, in the middle of such an unstable process, valves on one side of the combustion chamber are opened, and the burning fuel is allowed to break outside – yes, we can not expect a very even engine performance.
Still, despite all the problems, the engine performance subjectively is average. Largely, the increased idle help (the flywheel muffles the unevenness – the higher the RPM, the more efficient its performance). So my short answer is that you have to start to worry at the moment when you feel significant unevenness or shivering of the engine (misfires).
ISTA also warns that during the heating up of the CO catalytic converters, the performance of the engine is more uneven:

Unfortunately, BMW AG shares no information on the level of allowed unevenness. Zero data – as usual!
Below, I placed two examples with the normally performing B58 engine Rough run menu Expert mode.
First example: the engine temperature is +20 C (cold start on summer morning). On the X-axis, the timeline corresponds to the beginning of the star.

As we see, during the first several tens of seconds, the unevenness of the engine reaches around +/-15% (sometimes even 20) units.

After around 70 seconds, the heating of the CO catalytic converters stops. The idle is gradually reduced to 650 RPM. The performance of cylinders becomes more even, and the short-term unevenness does not reach over 4 .. 5 units.
Here, the performance of the engine after 5 minutes on idle.

Performance of the engine after 10 minutes on idle. The temperature of the coolant has reached 80oC. The indicator of the oil temperature in KOMBI starts to move.

Second example: repeated start of the warmed-up engine. On the X-axis, the timeline corresponds to the beginning of the start.

As we see, during the first 60 seconds (while the CO catalytic converters are warmed up), the short-term unevenness reaches +/-10 units. When the heating of the catalytic converter stops, the evenness of the engine performance upgrades swiftly.

According to the character of the unevenness, it looks that as soon as the heating of the CO catalytic converter is finished (and – by the condition that the engine has reached work temperature), DME switches on the updating of the injectors individual adaptations (prognosis of the mechanical efficiency). We can clearly see that the quality of DME “guessing” the ignition corrections of each next cycle increases: efficiency changes of each cylinder are not an “up-down/zigzag” line (as, for example, by heating the catalytic converters), but a line of very “soft” changes.