Fuel System
WARNING: This page is about a different car, the 2003 Oldsmobile Aurora. However, it is still accessible from the selected car via links, so may be relevant.
Check for leaking fuel injectors. See FUEL INJECTOR BALANCE TEST WITH TECH 2 or FUEL INJECTOR BALANCE TEST WITH SPECIAL TOOL under FUEL SYSTEM in SYSTEM & COMPONENT TESTING - 4.0L AURORA article.