At idle I am at 15% on bank one and 17% on bank two.. At running speed the both fuel trims tend to center at 0%. Truck has 277k on the clock. Normal? What should I try? Thanks.
Either air is sneaking into the intake or the MAF sensor is lying about how much air is entering the intake.
So, either a vacuum leak or dirty MAF would be my first guess.
Check for vacuum leaks. Use a can of carb cleaner to spray vacuum lines, connections, and around where your intake mates to the cylinder head, while the vehicle is idling.
Listen for idle changes.
Nothing? Then clean your MAF sensor with MAF cleaner and while you're at the auto parts store pick up a PCV valve if you haven't replaced it in a while.
As it turns out is was a bad PCV causing the vacuum leak. Good advice. Cheap fix. Fuel trims at idle are back to normal and morning cold starts are much smoother. Thanks.
I don’t know Chevys extremely well but generally spec is 100% +/- 10% so your not to far off but slightly on the lean side you could just check for intake or exhaust leaks though with those readings will be very minimal otherwise possibly an O2 sensor or mass air flow or map sensor just getting slightly dirty or worn out with those miles but again like I said you aren’t super far off
