Frank Jul 30th, 2003 Link
There is another very interesting fact acknowledged by the BMW engineers during Phobol's visit there. It appears to be that the stumble or engine stutter tends to become more pronounced when the gas tank is about 1/8 to 1/4 full, and the ambient temperature is above 90F degrees with humidity factor in the 30% to 90% ranges.
While I continue to say that software version update 33.2 has been very clean so far for the Cooper CVT, this week I have experienced the engine stutter (RPM's suddenly dropping while the car is idling in “D” mode from 1,100 RPM to 500 RPM and then bouncing back) meanwhile the gas tank was at or below the 1/4 full mark and the exterior temperature was 95F+ and humidity around 85%.
This problem doesn't happen when the gas tank is full or at least at by the 1/2 mark or either morning or night when both ambient temperature and humidity are lower than in the middle of the day.
To me this is not a problem with the sensors in the car. All the sensors in the car do is to collect data and send it over to the main EMS2000 Siemens DME for further processing. There is definetely a glitch in the software because when the gas tank is low perhaps some sort of vapor lock forms in the tank, the sensors collect this information and add to the mix the high ambient temperature and humidity and bingo! You get a sudden stutter.
I think this is the reason why these problems become so much more evident in warm weather and places with naturally hot climates.
Very interesting findings…stay tuned.
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