Sounds like it could be a oxygen sensor or fuel sensor. If that is the case, then the senors could be telling the ECU to put more gas into the engine than nessasary making the fuel to air mixture very rich which will waste gas.
It could also be a dirty or corroded fuel injector and fuel filter. If the injector is dirty or corroded, then the spray will not be uniform, resulting i bad internal combustion since the un-uniform spray can lead to bad fuel/air mixture.
