The glow plugs are only active at cold start of the engine. After that, combustion happens by compression and temperature of the mixture only. Thus glow plugs are probably not at fault. There are various other components to be checked. Your air filter may be in bad need of change. Your fuel filter (pre- and/or main-) may be clogged. Your fuel pump may not work optimally. Your injection pump may be out of tune/adjustment (delivery time and various other factors). Your injectors may be dirty or even some clogged (if not too bad, this condition can sometimes be remedied by a fuel additive for diesel engines that cleans injectors, but it takes time. It doesn't need many oversized particles to clog an injector). The engine could need a "valve job," which is necessary every few years. Worse, the cylinder seals might be worn down, lowering the engine compression. There could possibly be a combined effect from two or more components. For locations of these components, for checking them and getting them in order, you may want to consult the Chilton Mercedes repair book, or http://mb.braingear.com for the original MB repair and maintenance manuals, or download one at www.manualsunlimited.com. Http://dieselgiant.com might have useful information as well. I find a combination of the knowledge provided from these references (and as many others as I can obtain) is most helpful. True, a diesel engine is rougher in idle than a gasoline engine, but certainly not that much rougher as you describe. There is a simple idle adjustment in the engine, which can fix a rough idle with a twist of a screwdriver. Under load, a diesel engine in fairly good shape actually runs quite smoothly.