In theory, cracking the cap nut loose on the pump should leak fluid if the pump is working right. But I presume your mechanic would have checked that.
A diagnostic scope check is in order. You can literally see the effect of rich/lean on the scope for each cylinder.
If your mechanic doesn't know the system well, it's unfortunately time for a more seasoned mechanic for this.
Here's some good tech info...
http://www.pelicanparts.com/techarticles/911_MFI/TipMFI.htm
-CTH