Used engine installed, now getting code 118001?

First engine blown. Had new used engine installed and now get code 118001, mixture too lean. The mechanic says this has nothing to do with the engine installation so is not covered under my warranty. Wants to charge me an additional $1,300.00 to look for leak. The car runs, doesn't misfire or stall, just has loud rough idle and has poor acceleration. Car does not overheat. Sometimes white smoke from exhaust but not always.