I've got a 2006 Chevy Silverado with a 4.8l V8 and I've seen many a message around the P0449 error code but I have a question about symptoms other than the Check Engine light.

It all started with a "check or tighten gas cap" message on my dashboard. After ensuring it was tight, i replaced the gas cap and the message stopped appearing but the light has not gone away. I brought my truck down to the local Auto Parts shop and the ran an OBD II scan and told me I have error code P0449 and I should replace the Purge Valve Canister (ACDelco 214-1105 Valve Assembly). I still have the Check Engine Light on AND I more concerned with this other issue (This was happening BEFORE I replace the Purge Valve Canister). My truck seems to have a flat spot somewhere between 1500-1800 RPM. Whatever equates to somewhere between 53 and 56 MPH in top gear. With light throttle the engine rumbles so bad it feels like I am going over the rumble strips on the side of the highway. If I give it more throttle it gets past this point and seems to run fine.

Is this behavior typical with a P0449 error code or is there something else going on that I am missing?