BMW Repair: cars cuts off in heat, coolant temperature sensor, gas milage

Question
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Followup To

the gas milage is about 30 per gallon.  i check this every month or so.

Question -
First, thanks for the service.  I know you're not a VW person, but i think the question is general.

1990 VW Jetta, Manual.  190K miles.

when driving on a hot day in traffic, the car will cut off - maybe 2 to 3 times during a 25 miles and 45 minute freeway drive.  before cutting off it will sputter momentarily if moving, or simply cut off if stopped.  Each time i'm able to restart the car after about 1 to 2 minutes and i'm on my way.  if it's a hot day and not traffic, i have no problems.  if it's a cold day and a lot of traffic, no problem.

question - is there 'something' that causes this behavior?  vaccum, filter, etc?  Thanks.
Answer -
This is not an easy problem to diagnose it could be related to many different fuel injection components that. I am not familiar with on this model, I think your car uses CIS?  If so it could be a air density gauge, too much/little pressure in the tank/lines (vapor lock) or even a coolant temperature sensor, the best way to dignose would be with a sun scope and an exaust gas analizer.  How is the gas milage on this car? Close to when it was new?  Your car may be running too lean or too rich.
-cheers

Answer
I don't know a lot about this new of a model however I do know that it is possible to get live readings from every component enough to know what component is operating out of range. Seeing as it was sent to a dealer, I'm sure they have tried putting the scanner on it. Rough starting should not be caused by carbon buildup unless it somehow is clogging the valve or injector. The fact that it doesn't start or idle says the problem is most likely in the fuel system. Have them run a 5-gas analyzer on it to get the actual exaust gas readings, this should tell you if it's mixture related, or ignition related. Other than that I don't know of one single component that will cause this exact function, in order to diagnose, you must check every component and how the engine is handling the fuel to deduce the cause of the problem.
-cheers