How exciting!

MIL came on this evening, around the beginning of a 120 mile round trip. Car ran great, never skipped a beat, figured it was probably my on again, off again left cat code that comes and goes since I bought the car 2 years ago. But nope, today I have P0171 and P0174!

Did some searching, lean on both cylinder banks, so its got to be a common bit on the intake side. I tried spritzing brake cleaner around the intake tube, full load and part load breathers and the throttle body, nothing. Removed the intake and cleaned the MAF, reassembled. Put a bright light inside the air intake tube, couldn't see any light coming through the bellows. I think I can hear what sounds like a vacuum leak around the part load breather, which I snapped and had to repair with some shirk wrap when I did the cam cover gaskets, so it might have sprung a leak there, but it could also be the belts hissing. Guess I'll remove the belts and see if the noise persists. Would the part load breather trip these codes? Usually it seems like its more of an air intake tube problem than not, but of course just because others have had that, it doesn't follow that I will.

Other ideas?

 

Smoke test it. Fastest way to find leaks

 

Agreed. I had one around an injector seal. Spraying starter fluid and stethoscope didn’t help. Smoke test worked like a charm. I have a thread on how to built one.

 

Cool, thanks very much guys!

 

Old post bump for the solution in case this ever comes up in a future search. This issue was indeed my part load breather tube, my repair from a couple years ago failed and sprung a leak. I re-repaired it in the same way, cleaned the tubes, applied shrink wrap, held it together while trying (unsuccessfully) to not burn myself with the heat gun, viola! I'll replace the entire hose with a new one when I eventually have to pull the supercharger for a coupling replacement.