I'm having problems with my 2002 Oldsmobile Alero, 4cy. Few days ago, I turned on the car, the service light came on, it was idling very high, engine was chugging hard and exhaust smelled like sulphur. I drove it down the street and right back home because it had no power and then engine oil was low. When I pushed the accelerator, rpms went up and the car got louder but didn't speed up as it should. I've changed the oil, spark plugs and fuel filter but no improvement, service engine light is still on. What could be wrong with it?