Durango eating gas and blowing black smoke from tailpipe

Our 2002 Durango SLT plus had gone through a large puddle which ruined the engine. We then purchased a used engine with 68,000 miles, and our local mechanic installed it. After getting it back we've only gotten 6 miles per gallon instead of the 18-24 we used to get. In addition to his problem, our car kind of lunges, has black stinky smoke coming from the tailpipe, and takes more than a few times to get it started. We are at a loss here and don't know what to think. Any ideas or suggestions would be very appreciated!