Why does my 1998 c class mercedes automatic become sluggish when i first ?

Assuming its a diesel. Its more than likely a fuel problem rather than an injector or glow plug. It will be sucking air in by the fuel filter usually. Check none of the washers are squashed after refitting a filter. They have to be replaced when you replace the fuel filter and a lot of people dont bother. Sooner or later one or more starts letting in air. It starts up on first firing but that is only the fuel left in overnight. You recreate the vacuum needed by holding the starter till it fires or it just shakes and shudders on its own till the vacuum is re-established. Pennies to replace the washers. Small fortune to replace injectors and it doesnt fix the problem. Good luck.

Paul
July 2011