2001 PArk Ave Ultra pops when starting
 

Engine sometimes pops with a warm re-start (sounds like a small backfire at intake)? Also, very poor fuel mileage. Average city 11.5 to 13.5 mpg with sometimes sluggish performance. The EPA city mpg for my car is 16 mpg. Mechanic claims he can't find anything wrong. A friend has 2 PAU's, a 97 and 2003 - both get between 17.5 and 19 mpg city. I like my car but cannot afford to drive it often. Any tech tips would be appreciated. Thanks in advance.