2000 Sunfire stumble, black smoke

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?
--
Tegger


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Fuel problem -- way too rich. Since it's worse when cold, it might be a sensor. Or, it might be loose or worn wire (or wires) or some other problem with the computer triggering the injectors for the right amount of fuel when cold... Worse case, it might be the injectors themselves (that are electronically triggered, and also mechanical enough to go bad all on their own), but they can be changed without opening up the engine (don't know what they might cost). (Heck, the injectors might just be gummed up and sticking open, allowing too much fuel to flow. Try adding a can or two of "fuel injector cleaner" to the gas tank.) Even worse case might be the controlling computer's had a mental breakdown... The mechanic who went right to changing the coil pack is an idiot (and got to rip off your friend). Black smoke is too much fuel (too rich); when it happens when cold it's a choke problem (but with electronic fuel injection, there's no choke, just some electronics that richen up the mixture).

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I'd say.

Thanks. But I was hoping for some more specific information to pass on to my acquaintance.
I guess there's no history of this sort of thing with this model, so he's probably got something unique to his car.
--
Tegger


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