I wouldn't think thaat removing those pieces from the airbox would cause it to register lean either. I run an open K&N element and have never had it do that. You may have a vaccum leak that is allowing unmetered air in, even though it is running fine. Check the air intake hose for cracks around the throttle body, and along the vaccum lines, then reset the ecu and see if the code returns. If it doies you want to hunt down the problem, a lean mixture will lead to burnt valves. Also you might want to consider adjusting, the valves on the #4 cyl.(D-side center). You are right at the 20psi difference between cyls., which is the bottom end for being within specs.