so about 3 weeks ago i noticed my car was idling extremely high at about 1800 and normal all warmed up. and it continued like this for a couple weeks and i notice i was getting terrible gas mileage i was filling my tank from empty to full every 170 miles and was experiencing a huge loss of power couldn't even maintain 40 i was planning to take it in to the dealer the next week. a couple days passed like this and then my check engine light came on and now the car has more power than I've ever seen it have and I'm getting great gas mileage but my check engine light is still on. it is also very hard to start when the engine is warm. any ideas on what is causing it
2004 Pontiac vibe base with 5 speed automatic GT engine 770000 miles on it