Last week I tried posting this to alt.autos.gm and alt.autos.pontiac.
<http://groups.google.com/groups?as_q=sunfire&num=10&scoring=r&as_epq=&as_oq=&as_
eq=&as_ugroup=alt.autos.gm&as_usubject=&as_uauthors=tegger&lr=&as_qdr=&as_drrb=b
&as_mind=1&as_minm=1&as_miny=2007&as_maxd=3&as_maxm=12&as_maxy=2007&safe=off>

The responses were not really very revealing, so I'm re-posting it here
below, in the hopes some tech in this group might have some ideas.

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A guy I know has the above mentioned vehicle with the 2.4L four (the one
with two tailpipes and two mufflers) and a manual transmission. He's got
about 100K miles on it. There are no modifications to the car or its
wiring.

He knows next to nothing about cars, and I'm not familiar with GM vehicles,
so I'm hoping for some knowledgeable advice from the two groups I've
crossposted to.

When this car is cold, it emits black smoke, enough to discolor the bottom
of the bumper. Plus there is a stumble or hesitation when you press the gas
pedal. His MIL is often on, and sometimes it even flashes, which I
understand to mean that there is an emissions problem that is severe enough
to damage the catalytic converter. There is no hesitation or visible smoke
when the engine has warmed up.

He has taken the car to an independent garage. We do not know what specific
DTC(s) were pulled from the engine's computer. We do know that there was at
least one code for a misfire; that's all he was told. In an attempt to cure
the misfire, the independent garage replaced some or all of the coil packs
(is that what this car has?). This did not help at all.

My acquaintance tells me the problem seems to be worse in damp weather, and
that there is the occasional cold start where there is no hesitation.

I did some Googling in the hope that this may be a common problem, but it
appears not to be. Any ideas?