No power and lot of soot

Hi!
After replacing the gearbox and differential, the car lacks power and produces a lot of soot. Gear shifting is smooth, so there are no issues with that. Has anyone experienced a similar problem? What could be the cause—could it be unrelated, or was a mistake made during the repair?

Range Rover Sport 2006

Check for air intake pipes that may have come loose during the engine being tilted to accommodate the gbox work.

Any problems before the replacement box?