I have an LS1 swapped into an old pickup (from a 98 T/A). The motor is bone stock and used to run very strong.

The truck sat for about 2 years. Before firing it back up, I pumped out as much old gas as I could and added fresh 91 octane.

The motor starts and runs, but it idles rough and slow (used to idle around 750 RPM, now its closer to 600). Sitting at idle feels like I added a bumpy cam to it, or something. Stable, but not happy.

It stumbles and hesitates below 2800 RPM. If I floor it while it is hesitating, it cuts all power and bogs. Above 3k RPM, it runs smoothly. Power is way down across the board - I bet it is putting out 200hp or so.

No codes on my OBD2 scanner. I would think if it were a misfire, I’d get a code with how poor it is running. O2 senors shows normal output. The truck is emissions exempt so rear o2s, cats, EGR, etc were deleted. No obvious vacuum leaks.

If this were a carbed SBC I would have already replaced the cap and plugs/wires, but I would think I’d get a code with this? The tune is from a now defunct swap oriented company. Can tunes eliminate codes or raise the window for them?

My plan is to first replace plugs and wires (plugs are of unknown age and probably needs replaced anyway) to rule out the easy stuff. Then injectors & fuel filter? Then coils? Should I replace all of the injectors or try to clean them?

I feel like this should be straight forward but its been more than a decade since I worked on cars regularly, and I dont understand EFI other than the basics.

Thank you!