Ok to start out I have an 06 LT auto, for the past little while I've had the check engine light on, I had them checked and it said MAF sensor, knock sensor, and gas cap (this one I'm curious about) and since then the cel has been on constantly but the car has been running fine then a week or so ago the car started dipping rpms when I would slow down to a stop (15ish down to 0) occasionally dying, but once I came to a complete stop it would even out and idle fine, I also noticed a decrease in gas mileage. So today I got a new MAF sensor, installed it and still kinda having the same problem but it doesn't seem to be as bad now. I plan on replacing the gas cap and knock sensor as soon as I get some money, but could those things cause the problem I'm having? My history with cars screams vacuum lines to but I'm not in physically able shape to hover over the engine for extended periods of time, so before I do I kinda wanted some opinions and maybe tips of where to check. Any tips or input on this would be greatly appreciated. Thanks in advance!

 

Gas cap cel could also mean you have a big evap leak,there is no sensor for the gas cap being loose .
Its just a cel created for a sizable leak when a pressure check is done on the system at start up.
In a lot of cases it is the gas cap thats why They created the cel.
I would be thinking vac line leak ,concentrate on the line for the purge selinoid first but it could be your leak as Well if it is not closing all the way.This cuould cause random fumes getting into the throttle body and throwing your idle off.

 

....which would set a flow during non purge code.
It has nothing to do with P0440.


That code you have is 99% gas cap.
Did you clear the code after replacing those parts?