Big boost leak = high fuel trim?

[BIGGSIE]

My 90' TSI fuel trim is at 118.7% out of a possible 120% so I was doing some diagnosis. I found that I have a very large boost leak and it seems to be coming from the intake manifold gasket. I hook my leak tester up to shop air (140psi) and on full blast it will only get up to about 5psi on my guage. I can hear the leak but have not yet been able to locate it. My question is why does it run good? And why does it build boost good? I have a 95' TSI with a small leak at the TB and another small leak from the cheap ass plastic BOV. With the tester I can build good boost, that car also reads high fuel trims and sets a CEL but it has no power and sucks gas like a mofo. Are the 2G cars more touchy than the 1G's?

The way I see it:
Boost leak = lean at idle and rich under boost.
Does anyone agree with this idea?

 

[kpt4321]

Originally posted by BIGGSIE
Boost leak = lean at idle and rich under boost.

Yup, that's correct!

Ad idle, the leak will suck in more air, so you don't get enough fuel because the MAF doesn't see all the air.

Under boost, you will be rich because you're getting too much fuel, since some metered air is escaping.

Because of this, it is fairly easy to spot a post-MAF boost leak, if you know what to look for. When the car transitions from vacuum too boost, it will stumble, or bog a little.