Had the same problem with my 240 a few times, changed the main fuel filter and it cured it. Also worth checking your air filter and, obviously, your in-line fuel filter. If you are having problems with alge in your fuel, the filters can clog up over time and simply starve the engine of its power. Don't forget there is a filter in the bottom of your fuel tank too, but that is mainly for rock sized particles, but if your in-line is badly clogged it might be worth pulling the tank filter.

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'76 240D-Sold
'78 240D-Sold
'85 300 SD, 165K-Sold
'88 300 TE, 165K-Sold
'64 Porsche 356C Cabriolet- under restoration
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'94 320E Wagon, 74K Miles-128K Miles JUNKED
'06 E350 Wagon, 84K Miles
07 SL550, 14K Miles