Vac leaks are very hard to find on these systems b/c many can be Internal leaks that you will never see.

..the most common [ and I am only giving you these in probability rank b/c of known failure rates..I am not saying these are the problem] on these systems are Purge valve , egr valve , and Switch-over valves.

Purge is very common and an also mentioned on Alldata..they get crap in them from the canister and do not close/cycle well, thereby causing lean conditions.
Egr is same deal..they get dirty and do not snap shut...a hand vac test for this . The others can be check by pulling off every hose and checking for cracks or loose fitting.. I usually take a pair of sharp dykes and take every hose off, cut of and inch , and press them back on wit a little squirt of WD-40
On SOV's with plastic lines , I simply replace the plastic with rubber.

If you engine is running nicely, with good gas milage and nice power , you are probably fine and just need the extra % of adaptaion....but check these things anyway cuz they are free and should be part of general maint. anyway...........just like air filter and MAF cleaning.

__________________
A Dalton