Engine light was on, gas mileage dropped to around 7. code PO171, fuel system lean. Replaced o2 and map senor, reset computer and mileage went up to 18. Everything okay for 1 day, then engine light on again and mileage dropped back to 7. Now what can I do? No vacuum leaks I can see. 5.2 has 10,000 on it since rebuilt. Thanks for your help