I have a 1993 300D 2.5 mercedes that has no power under pressure. Going up grades or when I need to step down on it. Have put new vacuum pump on and it does the same. Starts great and sometimes runs great when it is cold but when it warms up it looses power again. Do you have any ideas. A lot of this year and model have this problem but have never seen any answers.
When's the last time you change the primary and the secondary fuel filter? This could also be caused by a restriction in the return line of the injection pump, when you opened up the fuel cap is the tank under extreme pressure? Also with 250,000 miles is due for timing chain to be replaced if the chain and sliders are worn the injection timing will be incorrect and cause it to be sluggish.