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?