The other day I noticed my esv idle's kinda low. It's been sitting for about 2months, anyways it idles about 600rpm(normal???), but when I put it in gear it drops kinda low. When its in drive I can give it some has and when I let off the idle drops low and j think the truck gonna stall. It did this before and lived with it but now I'm selling it and would like to fix this issue. What you think it can be. 2003 escalade esv, Idk when the last tune up was as well as fuel filter change.