1980 300D car only goes 5 mph when steping on the gas pedal on 1990 Mercedes-Benz 300D

Rookie cbe0621eac06868b3efe0d8d1d3611e23c60d3114864ea2ec19a68cfbd3eebab
The car lost power when driving and was only going 5 mph. What may this be?
(1) Answer
Have exhaust checked for back pressure probs. Have fuel pressure and volume checked. Have all this done at qualified shop.