recently i work in a workshop has overhaul an engine (mercedes E-class 240,V 6 engine)

when the engine is fixed in the car, it is difficult to start the engine...when the engine started, we loaded the engine but the engine don't have respond and when the engine respond, there is no power ...it looks like no vacuum when load the engine, the brake pedal is hard!
the engine timing has confirmed correct, the fuel system ok , compression ok , no fault code when diagnose the engine..

what things else is the problem cause the engine to run like this???