1978 240D black puff of smoke when starting

[SMG78]

Just got my car back from an incompetent repair shop. It went in because the glow plug light didn't go on and the car wouldn't start. I've been on the forum for this problem as well as fighting the bill and will get the car checked out by another mechanic.

New problems have developed since getting the car back last night.
--When I start the car, a black puff of smoke is emitted from the tail pipe leaving a big black oval on the the street.

--The mechanic said press the gas pedal down while waiting for the glow plug light to go off and then start the car. When the car turns over, the engine doesn't sound so strong at first and sounds as if it may die down. It takes a minute or 2 for the motor to sound strong. The car has a rebuilt motor that is 19 months old. Before it went into the shop, the car turned over and the engine sounded strong.

Thank you in advance.
__________________
1978 240D
1979 300SD (in garage not running)