Check for vacuum leaks at the exhaust manifold, and/or the right side of the inlet manifold, and/or the injector o-rings.
Believe it or not, a brokeass smoke test might be enough to detect it. Smoke a cigarette or a cigar (or something else) and get as much smoke as reasonably possible into the intake while the engine is running. You may very well find the leak with your eyeballs.
Then again, a talented mech might find the leak with his/her earballs.