[s-cars] cold weather and fuel question

[forrest bradshaw]

once in a while when I start my car after sitting for nearly 24 hours in temperatures around zero degrees, it starts the same as usual, but when I put it in reverse and apply enough gas to break free from the snow, I see black smoke out my tailpipe, this only lasts for a matter of seconds, Is this normal for such cold temperatures.  
Also, I have been getting around 18-20 max mpg with very modest driving since october.  My car also tends to idle around 1000 rpms at times, could these things be related, could my o2 sensor being going out on me?
car warms up to normal operating temps within 5 miles.
thanks in advance
Forrest


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