This happens to my dads car often too. Here are a couple of things that could cause this to happen.
1. You may need to replace the oxygen sensor. Over time, the sensor gets covered in oil ash and it reduces the sensors ability to change the oxygen and fuel mixture. A faulty sensor not only reduces gas mileage, it also increase emissions.
2.You could have a loose or faulty gas cap
3.You may need to replace spark plugs or wires
4. You may need to replace the mass airflow sensor
5. You may need to replace the catalytic converter
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