2003 Ford Taurus idles rough and has poor fuel economy

2003 Ford Taurus 3.0 V6_mileage: 48,000. Several months ago our Taurus city gas mileage dropped from about 23 mpg to 16 mpg. The highway miles are still close to 27 mpg. I have had it at two dealers who say no codes show. They replaced the IAC and fuel pump, with same results. When started even in warm weather the car jumps to over 2,000 rpm and remains for up to one minute. Seems to idle rough at about 800 rpm bouncing + or - 100 rpms. At stop lights it wants to idle high sometimes. Is this some type of vacuum leak? Thanks.