P0171 P0174 LTFT 24% at Idle

[1967ls2]

I'm throwing P0171 and P0174 codes on my 67 Camaro with an LS2/T56. The engine is stock with a CAI and headers. The car idles and runs fine under all conditions. I checked for vacuum leaks but found nothing obvious. I was checking out the fuel trim readings and note that at idle the LTFT for both banks are at 24% (which I understand is the threshold for throwing the codes). When driving they decrease but always return to 24% at idle or on decel. The STFT seem to fluctuate depending on load normally somewhere +or - of 10%. I pulled the MAF and cleaned it but that did nothing. I did notice this afternoon that a driver side header bolt was missing, closest to the firewall. I'm going to replace it, but doubt that is the source of my problems. My limited understanding of STFT and LTFT is that the ideal number would be closest to 0% which would indicate a good AFR of 14.7:1. I understand that the STFT will bounce around depending on load, but the LTFT should be closer to 0% more consistently. I am still thinking there is a vacuum leak somewhere which is causing both banks to read 24% for LTFT at idle, just not sure where else to look. I guess making my own smoke tester would be a logical step to rule out a vacuum leak.

Any thoughts on this issue?

[TBART70]

Long read might help

https://www.motor.com/magazine-summa...iagnostic-time

What type of air intake/ air filter set up is there?

Any air leak after MAF will skew numbers, any exhaust leak before O2 sensors will also cause issues, engine vacuum leak also as you said.

Intake vacuum leak can also be found by using water to see if sucked in/ we usually use brake clean or carb cleaner to see if idle changes, and if the O2 sensors go rich when sprayed with brake clean. Hitting with brake clean is a quick way to see if O2s move and go full rich or have a problem. same thing with creating a vacuum leak and seeing if they go full lean.

We do this on stock vehicles that are not Hot Rods so be careful if using solvents/

[1967ls2]

I was actually able to locate a vacuum port on the back of the intake that was unplugged. I put a rubber cover over it and it resolved the CEL. The odd thing is I never had these codes before and have no recollection of ever having put a cover on that port before. I guess one could have been on there and simply worked itself loose and then started throwing the codes.