It may have an O2 sensor that is sticking rich. The computer pulls fuel out trying to get to the O2 to read lean but it doesn't switch. You need a scan tool to look at the fuel trim and see what its doing.
The cheap way to get a scan tool is to download the torque app (basic app is free, full blown app is $4.99) from the Google Play Store. then buy a $7 bluetooth dongle from Amazon, I believe number for the dongle is Elm 327. it does a lot of other things as well.