Fire the mechanic and, as Cam suggests, make sure an actual tachometer is used. Diesel engines can run away if they suck in too much
fuel oil past bad rings and other problems, somehow either a governor is out of whack, or there is too much
fuel and air getting into the engine.
A mechanic shouldn't be baffled by that. the guy pretending to be a mechanic, owes you a refund and a beer.