99 Vortec high positive LTFT (>15%) both banks. No CEL

[Kran]

Not sure if this is a sudden issue or not. I've had the truck seven years. Truck has a small miss/shake at idle but will happily idle all day. Drives fine. Plugs and wires done within the last two years. Cap and rotor this year. Both banks are showing very high LTFT while driving, usually around 15-18% but bank 1 shot up to 20% briefly while I glanced at the OBD2 reader on the highway. Going completely off throttle both banks shoot back down to about 6%. Haven't checked trims while idling, forgot. There is no CEL. I have used a bottle of Techron fuel cleaner once a year, the last two years. Unsure when MAF was changed, I never did it. Fuel pump is a Delphi and replaced a few years ago along with filter. Unsure of spider injector state or if it was ever replaced with newer style. Intake gaskets done about five years ago.

Thoughts on where to start troubleshooting, if anywhere? Or am I just worrying for nothing? Thanks.

 

[EJ_74]

I would think some variation while driving is probably normal. Wouldn't hurt to change the O2 sensors though if they are old

 

[Cadillacmak]

Check your fuel pressure. I am was having the same exact issue, replaced fuel filter and checked pressure. Its low, now waiting on the new pump (because sender is bad too) to arrive, I think the regulator is bad too but haven't got back to that truck yet.

 

[Schurkey]

Yup. First thing to do is replace old O2 sensor(s). Second is to verify fuel pressure.

My limited experience is that old O2 sensors tend to drift towards "lean" more than they should. The computer compensates by adding more fuel. Which is why replacing old O2 sensors tends to improve fuel economy.