1998 Infiniti I30 mass air flow sensor?

My car's engine light has come on. I thought that I needed a tune up because of how it was running. But that didn't work. It is burning an extremely large amount of gas. I literally have to put about 75-100 dollars per week in gas in it to ride for the week. Whereas I normally could ride for about 30-40 a week. It also has a foul order sometimes and it has black smoke coming out of the tail pipe when it stalls ocassionally. I was wondering would it be the mass air flow sensor or the fuel injection line. I don't know but it is getting to the point where I can't afford to drive my car because I can't keep gas in it. Please help me!