Chevrolet Repair Questions? Ask a Mechanic for Answers ASAP
Spray some carb cleaner around the base of the manifold and see if the idle changes at all. If you have a scanner, you can monitor the fuel trims while you do this. Very good chance that your intake gaskets are leaking, causing a vacuum leak, hence the lean codes.
If that doesn't pan out, measure your fuel pressure, make sure its good, should be somewhere between 45-55 PSI, might be a good time to replace the fuel filter if it hasn't been done in a while.
Good luck and hope this helps.