The past few weeks I could hear a vacuum leak when I shut down my engine. Took a while to track it down, even with spraying MAF cleaner all over. The only time I got the engine to act up was when I sprayed under the supercharger shout. The leak it would seem was between the supercharger and the intercooler plate. I did some scans, even though the engine was running fine. No indication of a vacuum leak, other then the hissing sound. The scans showed my LTFT to be at +19, Hmm, that should be a zero. So what does that meen? I have unmetered air entering the engine causing me to run lean. So the engine is adding fuel to compensate for this. Hence the +19 on the long term fuel trims.
I got to work pulling off the supercharger
Then cleaned up the intercooler plate.
I found no indication of a vacuum leak. But I cleaned up the surfaces and used some red RTV to seal it good. Total time with me taking my sweet old time was 1 hour 40 mins.
Started it up and found no leaks. Hooked up the computer and reset the fuel trims. They seem to be setteling back in about -5 right now.
So, if you do a scan and find your LTFT way off, it could be a vacuum leak if it'* positive, and could be a exhaust leak if negitive.