It is probably doing what is called, "Dieseling". This is from carbon desposits on the psitons being large enough to stay hot and continue to ignite the gas in the combustion chamber which keeps the motor turning and gas coming into the combustion chamber. Since it is low RPM it does not take much fuel vapor to keep it running like this for a while.
You need to run some high quality fuel cleaner. BG44K is the only one that really will fix situations like yours. You have to go to a car dealership parts department to get it and it is $20.00 a bottle but it is worth it. You should run 2 bottles each on a full tank. You might experience the engine runnning oddly as depsoits are being removed. By the time you run a third full tank of gas you will be able to see if it solved or improved the problem. It will probably be enlough to take care of it. All the deposits will end up in the engine oil so you might want to change it after the third tank.