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.

very interesting.... I will use that at work.... thanks.

Dan,
your post inspired me to go right out and do another live scan. My LTFT was 140. Don't know what the LTFT cl is but it was 16. This was at idle with the emergency brake on and transmission in reverse. I had noticed that the scantool would not pick up the miss in park but would show a miss on cylinder # 4 under load. I sprayed carb cleaner around the base of the supercharger and LIM and got no response. When I got to the base of the upper EGR tube the engine shut off. I restarted and sprayed it again, it shut off again. I had to JB weld the upper end of the EGR tube a while back due to a crack. So, tomorrow I will be off to the JY. I saw a Buick Park Avenue Ultra yesterday that was missing the supercharger but the EGR tube looked to be intact. Didn't have my tools with me, just happpened to be in the area. I will post back with an update. Meanwhile if anyone needs fuel injectors or any such thing I may be able to pull them over the next couple of days. Oh, one more thing, with the tranny in park the ks was at 3, in reverse with the emergency brake on ks was 171. ST fuel trim fluctuated between 124 and 131. By the way, the Buick is a 94 I think.

A cracked EGR tube is a big vacuum leak. That will cause you to run lean and really raise your LTFTs

Dan, did you also pull the IC and check for a leak from the IC to LIM?

It wasn't leaking there, but I did pull that and seal it good.