I have been having this code come on for a while now. I replaced my gas cap with a GM replacement. I am not having any performance problems with it at all. What else besides the gas cap could be causing a small evap leak?

 

From the Helms service manual, 2000 F body:
A loose, missing, defective, or incorrect fuel tank cap
Incorrectly routed or defective EVAP system vacuum and vapor lines
A malfunctioning or damaged vapor canister
A damaged fuel sender O ring.

Could maybe be the purge solenoid, I broke mine (plastic where the lines hook up) somehow when installing headers.
Its on the drivers' side of the intake manifold, and has a line that connects to the intake behind the TB (vacuum source). Costs about $25 at Advance.
Curious thing is, mine never threw a code.
Good luck.