Car cranks but won'ts start

About a month ago, the car began making this noise that sounded like a siren. 20 mins aftr sounds began it stopped and a cloud of white smoke came from the rear of the vehicle. Mechanic stated the the turbos ahd gone bad and needed to be replaced. They have since been replaced, but the car will not start. It cranks, but won't strat. he has adjustd the timing, but can not find anything wrong with the vehice. Mechanic now thinks tha tthe engine control unit is bad, but wants to continue to trying to figure out what's wrong. he is alleging that the the car in backfiring into itself because the computer is sending a bad sigmal. Long sotry short, I have been without my car for over a month. He doesn't seem to know what's wrong and has not given me an anticipated completion date. Please tell mw what you think the issue is so I can run these things by the mechanic. Thank you!

I need by car ASAP.