Black smoke and metalic burni g smell
There could be multiple problems with your Fiat, and from afar, nobody can accurately diagnose the problem(s). I strongly suggest that you drive to a nearby mechanic’s shop… after you have verified that nothing is actually burning.
Are there any warning lights lit up on your instrument panel? If so, which lights?
Instead of driving, it might be a good idea to have it towed to a mechanic’s shop.
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If the black smoke is coming out the exhaust, it means the engine is running very rich. And if that’s what’s happening, what you might be smelling is the catalytic converter burning up.
Tester
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There were multiple problems with Fiat 500s when they were brand new! 
Clearly that has only gotten worse!
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I was trying to be diplomatic, but… you are correct.
After 12 years, a flawed design certainly doesn’t get any better.
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